Make FixAll APIs for ContainingType/Member scope public

@jmarolf Yes, these APIs are intentionally public. Otherwise, public clients such as MS.CA.Testing library, would be unable to create a FixAll provider that supports these new FixAll scopes. We can certainly keep them internal in this PR and have a separate PR to just make these APIs public.

_Originally posted by @mavasani in https://github.com/dotnet/roslyn/pull/59989#discussion_r821311317_