Make it easier to work with external protocols & example data

[ajfabbri]

• Add cli arguments to generate.py script to specify protocol and documentation directories.
• Start the process of creating a proper python package out of this. Still needs work to be useful as a pip install target.
  • Move python source code from avro/python to interface-gen
  • Add pyproject.toml package metadata and separate distribution readme. Uses hatchling as build backend to create packages for pypi.org etc.