Add more gRPC practice projects

Our fellows should have written so many gRPC servers and clients that they can spin up a new one from scratch in 15 minutes.

Right now we only have one gRPC project, and it lacks:
1. Writing the proto definitions from scratch
2. Hosting multiple services (both on the same port, and on separate ports)
3. Single binaries acting as both clients and servers

We should bulk out the gRPC experience a bit with some repetition so that by the time they're implementing raft, this stuff is second nature.

Start by inspecting the repository's existing gRPC project to understand its structure and practice format. Expand the exercises to include writing proto definitions, hosting multiple services on the same and separate ports, and binaries that act as both clients and servers; done means these scenarios are represented as runnable practice projects.

Written by the indexing model from the issue text.