[csharp] reintroduce base_namespace experimental option to C# (with a patch)

[jtattermusch]

Reintroduces patched version of #32636 (which was reverted in #32957).

Together with cl/542843305, the internal build should work fine.

Supersedes #33310 (since a patch is also needed).

[jtattermusch]

@tonydnewell

[jtattermusch]

The internal build is passing, so I'm going the assume that my fix worked and this PR is ready to merge.

[felschr]

I'm not entirely sure what happens here, but with protoc's base_namespace option I can specify base_namespace= (yes, without a value after the =) and it'll generate the code into a folder structure instead of putting everything into the root directory.
I expected the same behavior for grpc, but it still tries to put all files into the root directory, which in my case causes issues with duplicate file names.

Since GRPC_CUSTOM_CSHARP_GETOUTPUTFILE() already handles the base_namespace.empty() case perhaps the check in grpc just needs to be removed?:
https://github.com/protocolbuffers/protobuf/blob/1bfcedaf1280923f36e52a6fa0a5645ab63568f0/src/google/protobuf/compiler/csharp/names.cc#L137-L153

[felschr]

Just found the empty string behavior in protoc documented here:
https://github.com/protocolbuffers/protobuf/blob/1bfcedaf1280923f36e52a6fa0a5645ab63568f0/src/google/protobuf/compiler/csharp/csharp_options.h#L63-L67

So, essentially when base_namespace is explicitly set via the CLI the variable base_namespace_specified is set to true which in turn causes GetOutputFile() to be invoked with generate_directories set to true.

To match the behavior, grpc should also check if base_namespace was explicitly set and use that in the conditional, e.g.:

Suggested change

	if (base_namespace.empty()) {
	out_file = grpc_generator::FileNameInUpperCamel(file, false) + file_suffix;
	} else {
	out_file = GRPC_CUSTOM_CSHARP_GETOUTPUTFILE(file, file_suffix, true,
	base_namespace, error);
	if (out_file.empty()) {
	return false;
	}
	}
	if (!base_namespace_specified) {
	out_file = grpc_generator::FileNameInUpperCamel(file, false) + file_suffix;
	} else {
	out_file = GRPC_CUSTOM_CSHARP_GETOUTPUTFILE(file, file_suffix, true,
	base_namespace, error);
	if (out_file.empty()) {
	return false;
	}
	}

No need to removing this check entirely to retain backwards compatibility. See https://github.com/grpc/grpc/pull/32636/files#r1148360058 for some notes on that.

[felschr]

I've created a new issue for this: #34113