Introduce Issuer handling in the Authentication stack (#248948)

Mostly plumbing... this enables:
```
vscode.authentication.getSession('microsoft', scopes, { issuer: "https://login.microsoftonline.com/common/v2.0" });
```
And the respective API for an auth providers to handle it being passed in.

This props up work in MCP land which needs a way to map an issuer to an auth provider... but I certainly see utility outside of that space.

Fixes https://github.com/microsoft/vscode/issues/248775#issuecomment-2876711396

extensions/github-authentication/src/github.ts

@@ -137,7 +137,17 @@ export class GitHubAuthenticationProvider implements vscode.AuthenticationProvid
 
 		this._disposable = vscode.Disposable.from(
 			this._telemetryReporter,
-			vscode.authentication.registerAuthenticationProvider(type, this._githubServer.friendlyName, this, { supportsMultipleAccounts: true }),
+			vscode.authentication.registerAuthenticationProvider(
+				type,
+				this._githubServer.friendlyName,
+				this,
+				{
+					supportsMultipleAccounts: true,
+					supportedIssuers: [
+						ghesUri ?? vscode.Uri.parse('https://github.com/login/oauth')
+					]
+				}
+			),
 			this.context.secrets.onDidChange(() => this.checkForUpdates())
 		);
 	}