GitHub Enterprise Auth improvements (#165082)

1. Namespace secrets based on the value of github-enterprise.uri to support "multiple separate GHES instances"
2. If the setting value disappears, continue using last set value. Fixes https://github.com/microsoft/vscode-pull-request-github/issues/3992
3. Mark github-enterprise.uri as requires trust
3. Refactoring like:
  * UriHandler is handled in extension.ts and passed down everywhere since we can only have 1 instance of it
  * misc style (`private` usage, better `disposable` handling)