Revert ISB services

This cleans up some issues identified in the nullability default change
PR (#47961)

Expands #47903 to other calls to GetProxyAsync.

Update help service to account for all `TypeDeclarationSyntax` (namespace+record+struct) as partial type (#48095)

* Put generated files to disk:
- Add new command line param 'GeneratedFilesOut'
- Route new param through tasks with defaults
- Write out generated files when requested to do so
- Rationalize post generation to avoid uncesseary work
- Add Tests for targets and emit
- Add test to track the fact that we can overwrite files on windows in SxS cases