Add gRPC middleware support

[makarski]

syntax = "proto3";
package transform;

service Transform {
  rpc Transform (TransformRequest) returns (TransformResponse);
}

message TransformRequest {
  bytes payload = 1;
  string encoding = 2;
  string method = 3;
  map<string, string> attributes = 4;
}

message TransformResponse {
  bytes payload = 1;
  map<string, string> attributes = 2;
  bool success = 3;
  string error = 4;
}