feat: added user gRPC service

Contracts/Proto/user.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+option csharp_namespace = "JOBot.Proto";
+
+service User {
+	rpc Register (RegisterRequest) returns (RegisterResponse);
+}
+
+message RegisterRequest{
+	int64 user_id = 1;
+	string username = 2;
+}
+
+message RegisterResponse{
+	int64 user_id = 1;
+}