Add Nim benchmark

[kubo39]

repository: https://github.com/nitely/nim-grpc

[kubo39]

I tried and gotta this error.

(snip...)
34.84 stack trace: (most recent call last)
34.84 type_generator.nim(186, 32) protoToTypes
34.84 type_generator.nim(74, 45) protoToTypesInternal
34.84 proto_parser.nim(306, 46) parseProtobuf
34.84 proto_parser.nim(108, 22) parseProtoPackage
34.84 proto_parser.nim(76, 5)  tokenize
34.84 /app/src/server.nim(8, 13) template/generic instantiation of `import_proto3` from here
34.84 /root/.nimble/pkgs2/protobuf_serialization-0.3.0-ed8270a5f874af35c5e9c04b50020c8a27ba61f5/protobuf_serialization/files/proto_parser.nim(76, 5) Error: unhandled exception: helloworld.proto:28 Failed to lex '('
34.84   rpc SayHello (HelloRequest) returns (HelloReply) {} [CatchableError]
34.84        Tip: 268 messages have been suppressed, use --verbose to show them.
34.84 nimble.nim(304)          buildFromDir
34.84
34.84     Error:  Build failed for the package: grpc_bench

nim-grpc currently doesn't support a grpc plugin.