[Chronik] Build proto for python + HTTP 404 proto fallback

Summary:

1. Compile protobuf for python using protobuf_generate_python in CMake.
2. Add generated python modules to sys.path.
3. Add a 404 fallback for the HTTP server.

Test Plan:

1. ninja check-functional to generate python protobuf
2. Ctrl+C once tests start
3. ./test/functional/test_runner.py chronik_serve

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: kongpcmail

Differential Revision: https://reviews.bitcoinabc.org/D13213