This PR implements the gRPC versions of the JSONPlaceholder REST APIs for Posts and Users as specified in #40

Changes Made

• Proto Definitions: Created proto/posts.proto and proto/users.proto to define PostService and UserService.
• Build Configuration: Updated build.rs to compile all proto files from the proto directory.
• Service Implementation:
  • Implemented PostService with ListPosts, GetPost, CreatePost, UpdatePost, and DeletePost.
  • Implemented UserService with ListUsers, GetUser, CreateUser, PatchUser, and DeleteUser.
• Server Registration: Updated the main server to register and serve the new gRPC services.
• Tests: Added tests that verify all functionlity

/fixes #40 /claim #40