Backport #6759 and disable extension code gen for C#

[ObsidianMinor]

This fixes #6749, #6510, and #6509

(Backports #6759 into 3.10.x branch).

[ObsidianMinor]

I hope I did this patch right. @jtattermusch

[ObsidianMinor]

Yeah, I forgot this requires removing a lot more code to make custom options work. Try running kokoro again.

[jtattermusch]

LGTM.

[jtattermusch]

@rafi-kamal can we push a new patch release after merging this?

[rafi-kamal]

Sure, will do.

[rafi-kamal]

@jtattermusch I tried doing a release today, but it's blocked on https://groups.google.com/a/google.com/forum/#!topic/maven-users/XahhvcPnwfw due to a protoc error. Does C# protobuf depend on protoc? If not, then I can do a release only for C# without releasing protoc.

[ObsidianMinor]

Part of the patch disables certain C# code-gen so for it to be completely effective protoc should be included.

[rafi-kamal]

Release 3.10.1 to Nuget. I'll be OOO for a week from this afternoon so please reach out to the protobuf team if there is any problem.

On a side note, does anyone have instructions for rolling back released Nuget packages so that I can link it to our release docs?

[jtattermusch]

The new nuget is here: https://www.nuget.org/packages/Google.Protobuf/3.10.1

Ad "rolling back" packages: there is no such thing with nuget.org. You can only mark packages as "unlisted" in nuget.org UI (= they will be still there for those who reference that version in their project, but they won't show up in search results). To unlist you just log in to www.nuget.org and mark specific package version as "unlisted".

[jtattermusch]

@ObsidianMinor because this PR also has codegen changes, it sounds like we'll need to also pull the changes into Grpc.Tools nuget (which also bundles protoc binary) for the fix to be really effective?