Add VSCODE_PROXY_URI environment variable

This can be used by extensions to open a port and access it through the proxy.

It is available in the terminal as well.

This can be tested using printenv in the terminal and by using the
codeServerTest.proxyUri command through the test extension (copy it into your
extensions, use --extensions-dir, or symlink it).

This has e2e tests.

Index: code-server/lib/vscode/src/vs/base/common/product.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/base/common/product.ts
+++ code-server/lib/vscode/src/vs/base/common/product.ts
@@ -35,6 +35,7 @@ export interface IProductConfiguration {
 	readonly rootEndpoint?: string
 	readonly updateEndpoint?: string
 	readonly logoutEndpoint?: string
+	readonly proxyEndpointTemplate?: string
 
 	readonly version: string;
 	readonly date?: string;
Index: code-server/lib/vscode/src/vs/platform/remote/browser/remoteAuthorityResolverService.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/platform/remote/browser/remoteAuthorityResolverService.ts
+++ code-server/lib/vscode/src/vs/platform/remote/browser/remoteAuthorityResolverService.ts
@@ -7,7 +7,7 @@ import { Emitter } from 'vs/base/common/
 import { Disposable } from 'vs/base/common/lifecycle';
 import { RemoteAuthorities } from 'vs/base/common/network';
 import { URI } from 'vs/base/common/uri';
-import { IRemoteAuthorityResolverService, IRemoteConnectionData, ResolvedAuthority, ResolverResult } from 'vs/platform/remote/common/remoteAuthorityResolver';
+import { IRemoteAuthorityResolverService, IRemoteConnectionData, ResolvedAuthority, ResolvedOptions, ResolverResult } from 'vs/platform/remote/common/remoteAuthorityResolver';
 
 export class RemoteAuthorityResolverService extends Disposable implements IRemoteAuthorityResolverService {
 
@@ -20,7 +20,7 @@ export class RemoteAuthorityResolverServ
 	private readonly _connectionToken: string | undefined;
 	private readonly _connectionTokens: Map<string, string>;
 
-	constructor(connectionToken: string | undefined, resourceUriProvider: ((uri: URI) => URI) | undefined) {
+	constructor(connectionToken: string | undefined, resourceUriProvider: ((uri: URI) => URI) | undefined, private readonly proxyEndpointTemplate?: string) {
 		super();
 		this._cache = new Map<string, ResolverResult>();
 		this._connectionToken = connectionToken;
@@ -59,12 +59,17 @@ export class RemoteAuthorityResolverServ
 
 	private _doResolveAuthority(authority: string): ResolverResult {
 		const connectionToken = this._connectionTokens.get(authority) || this._connectionToken;
+		let options: ResolvedOptions | undefined
+		if (this.proxyEndpointTemplate) {
+			const proxyUrl = new URL(this.proxyEndpointTemplate, window.location.href);
+			options = { extensionHostEnv: { VSCODE_PROXY_URI: decodeURIComponent(proxyUrl.toString()) }}
+		}
 		if (authority.indexOf(':') >= 0) {
 			const pieces = authority.split(':');
-			return { authority: { authority, host: pieces[0], port: parseInt(pieces[1], 10), connectionToken } };
+			return { authority: { authority, host: pieces[0], port: parseInt(pieces[1], 10), connectionToken }, options };
 		}
 		const port = (/^https:/.test(window.location.href) ? 443 : 80);
-		return { authority: { authority, host: authority, port: port, connectionToken } };
+		return { authority: { authority, host: authority, port: port, connectionToken }, options };
 	}
 
 	_clearResolvedAuthority(authority: string): void {
Index: code-server/lib/vscode/src/vs/server/node/webClientServer.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/server/node/webClientServer.ts
+++ code-server/lib/vscode/src/vs/server/node/webClientServer.ts
@@ -294,6 +294,7 @@ export class WebClientServer {
 					rootEndpoint: base,
 					updateEndpoint: !this._environmentService.args['disable-update-check'] ? base + '/update/check' : undefined,
 					logoutEndpoint: this._environmentService.args['auth'] ? base + '/logout' : undefined,
+					proxyEndpointTemplate: base + '/proxy/{{port}}',
 					codeServerVersion: this._productService.codeServerVersion,
 					embedderIdentifier: 'server-distro',
 					extensionsGallery: {
Index: code-server/lib/vscode/src/vs/workbench/browser/web.main.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/workbench/browser/web.main.ts
+++ code-server/lib/vscode/src/vs/workbench/browser/web.main.ts
@@ -179,7 +179,7 @@ export class BrowserMain extends Disposa
 
 		// Remote
 		const connectionToken = environmentService.options.connectionToken || getCookieValue(connectionTokenCookieName);
-		const remoteAuthorityResolverService = new RemoteAuthorityResolverService(connectionToken, this.configuration.resourceUriProvider);
+		const remoteAuthorityResolverService = new RemoteAuthorityResolverService(connectionToken, this.configuration.resourceUriProvider, this.configuration.productConfiguration?.proxyEndpointTemplate);
 		serviceCollection.set(IRemoteAuthorityResolverService, remoteAuthorityResolverService);
 
 		// Signing
Index: code-server/lib/vscode/src/vs/workbench/contrib/terminal/common/terminalEnvironment.ts
===================================================================
--- code-server.orig/lib/vscode/src/vs/workbench/contrib/terminal/common/terminalEnvironment.ts
+++ code-server/lib/vscode/src/vs/workbench/contrib/terminal/common/terminalEnvironment.ts
@@ -388,7 +388,7 @@ export function createTerminalEnvironmen
 
 		// Sanitize the environment, removing any undesirable VS Code and Electron environment
 		// variables
-		sanitizeProcessEnvironment(env, 'VSCODE_IPC_HOOK_CLI');
+		sanitizeProcessEnvironment(env, 'VSCODE_IPC_HOOK_CLI', 'VSCODE_PROXY_URI');
 
 		// Merge config (settings) and ShellLaunchConfig environments
 		mergeEnvironments(env, allowedEnvFromConfig);