Allow _V2 protoc option

[Clement-Jean]

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 (

rules_swift/proto/compilers/swift_proto_compiler_macros.bzl

Line 39 in 86dc0f0

GRPC_PLUGIN_OPTION_ALLOWLIST = PROTO_PLUGIN_OPTION_ALLOWLIST + [

).

would it be possible to add it?