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