gRPC improvements, mTLS, and more

[mortendahl]

This PR does quite a bit:

• separates the protobuf services, Executor and ChannelManager; this captures the fact that the former is used by the orchestrator while the latter is simply an implementation detail of (one way of doing) networking
• makes it possible to use gRPC over mTLS
• cleans up that way networking is done, avoiding the need to access local buffers through gRPC
• renames Server to Worker
• renames KernelBasedExecutor to AsyncExecutor (since they're all likely to be kernel based, but we could have other non-async strategies)
• moves kernel implementations into isolated files
• shows how docker compose can be used to spin up an extra "coordinator" service that simplifies how examples can be executed (in python-functions)
• installs and uses watchexec to automatically restart workers have changes are detected to the source code

TODO:

• restarting workers using watchexec should be for development only; maybe separate dev-worker image?
• update all examples to match python-functions (see Update all examples to match python-functions #70)
• update relevant example READMEs
• mTLS for runtime as well

[yanndupis]

Hey @mortendahl, excellent work on this 💪 ! I don't have other comments than the one mentioned by Dragos.

As mentioned in your TODOs would be great to have all the examples matching python-functions example but it can be done in a separate PR. Thanks