refactor!: split AvroAttrsBridge into distinct methods

[rgraber]

Description: Split the AvroAttrsBridge into several different modules/classes to separate the schema generation, serialization, and deserialization phases.

JIRA: ARCHBOM-2102
GH Issue: #65

Testing instructions:
Usage described in How-to

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[robrap]

Nice. I like that the different modules keep a clear distinction between schema, serialization, and deserialization code.

[robrap]

Thanks Becca. I want to help wrap up this PR quickly, so if anything needs discussion, maybe we can handle that synchronously to land this more easily?

I added lots of comments, but many are suggested changes. If you like the change, I highly recommend you just click the "Add suggestion to batch" button to make your life easier, and then add a commit with all the changes.

Some of the test recommendation updates I made aren't really blockers. If they are quick, please update. Otherwise, you can either note that you are skipping and/or we could add a TODO for potential future clean-up.

Thank you!