Fully automate vendor maven updates

[TheTripleV]

Basically,

1. Check for update
   try:
2. Run scan headers
3. Build
4. Run Gen Imports
5. Rebuild
6. Create PR
   Except:
7. Create Issue

[virtuald]

This is a worthy goal, and definitely seems doable for in-season updates. Some things standing in the way:

• scan-headers doesn't modify pyproject.toml. Will want to use tomlkit for this, to ensure that existing format/comments are kept in the file.
• You forgot about create-gen --write... though I guess it's not strictly necessary. It would also be nice if create-gen could modify the existing yaml files.
• gen-imports doesn't modify __init__.py either, and there are some cases where imports are conditional. Would need a way to specify these.