[BulkBeing]

Make the sourcetransformer API Pythonic, the same as the mapper components.

• The handler no longer takes keys. Use datum.keys. The handler signature is now handler(datum) -> list[Message].
• message_to_drop() is renamed to to_drop().
• New SourceTransformAsyncServer wrapper with serve(), wait_ready() and async with support.
• Handler errors now raise in Python. They no longer panic in Rust.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.99%. Comparing base (2d585e6) to head (2d261f8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #386   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files          68       68           
  Lines        3656     3656           
  Branches      236      236           
=======================================
  Hits         3400     3400           
  Misses        190      190           
  Partials       66       66           

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