Issue Type: Feature Request

Currently we don't support signing into GitHub Enterprise accounts using the auth provider. The auth provider makes calls to get user information that point specifically to github.com.

Either the getSessions API needs a way to provide the enterprise server endpoint, or the auth provider extension should automatically do this detection and let the user choose when performing sign in. The auth provider needs to store what endpoint tokens are associated with and use this instead of always going to github.com. It should also indicate this in the UI.

VS Code version: Code 1.51.1 (e5a624b, 2020-11-11T01:11:34.018Z)
OS version: Darwin x64 18.7.0