feat: added user gRPC service

JOBot.Backend/Services/gRPC/UserService.cs

@@ -0,0 +1,16 @@
+using Grpc.Core;
+using JOBot.Proto;
+
+class UserService : User.UserBase
+{
+    public override Task<RegisterResponse> Register(
+        RegisterRequest request,
+        ServerCallContext context)
+
+    {
+        return Task.FromResult(new RegisterResponse
+        {
+            UserId = request.UserId
+        });
+    }
+}