Fix for https://github.com/grpc/grpc/issues/34113 - setting C# option
base_namespace to an empty string results in inconsistent behavior
Allow the `base_namespace` option in the protocol buffers C# gRPC plugin
to be specified with an empty string to make sure the plugin has the
same behaviour as the protocol buffers compiler.
Reverts grpc/grpc#32636
```
src/compiler/csharp_generator_helpers.h:25:7: error: no member named 'compiler' in namespace ...
src/compiler/csharp_generator_helpers.h:25:25: error: no member named 'csharp' in namespace 'compiler' ...
```
Added `base_namespace` experimental option to `grpc_csharp_plugin` as
this has been requested several times by
people not using `Grpc.Tools` to generate their code - see
https://github.com/grpc/grpc/issues/28663
Notes:
- it should not be used with `Grpc.Tools`. That has a different way of
handling duplicate proto file names in different directories. Using this
option will break those builds. It can only be used on the `protoc`
command line.
- it uses common code with the `base_namespace` option for C# in
`protoc`, which unfortunately has a slightly different name mangling
algorithm for converting proto file names to C# camel case names. This
only affects files with punctation or numbers in the name. This should
not matter unless you are expecting specific file names
- See
https://protobuf.dev/reference/csharp/csharp-generated/#compiler_options
for an explanation of this option
