Allow `_V2` protoc option

I would like to be able to generate V2 protobuf code (https://github.com/grpc/grpc-swift/blob/f3380d217ebd7f5a939115736bcc19fe1752fee9/Plugins/GRPCSwiftPlugin/plugin.swift#L73). I was able to do it manually like the following:

```
protoc --swift_out=. \
--swift_opt=FileNaming=DropPath \
--grpc-swift_out=. \
--grpc-swift_opt=Client=true,Server=false,FileNaming=DropPath,_V2=true \
proto/my.proto
```

but it seems the rules_swift doesn't accept this option (https://github.com/bazelbuild/rules_swift/blob/86dc0f046269b3001f6f20cec38342c03120a209/proto/compilers/swift_proto_compiler_macros.bzl#L39).

would it be possible to add it?

Start in proto/compilers/swift_proto_compiler_macros.bzl at the referenced compiler options list and compare it with the manual protoc command in the issue. Confirm that rules_swift accepts _V2=true for grpc-swift generation; done means the option is accepted through the Bazel rule and V2 Swift protobuf code can be generated.

Written by the indexing model from the issue text.