feat(python): support proto-only APIs

[jskeet]

Adds handling for "proto-only" APIs (by which we mean APIs without a GAPIC library configured). These APIs:

• Use the python and pyi protoc plugins, not gapic
• Generate to a slightly different staging directory
• Copy the .proto files as well as the generated code

This change configures these APIs during migration.

Fixes #3204.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.90%. Comparing base (e323d45) to head (8bfd87f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4075      +/-   ##
==========================================
- Coverage   81.93%   81.90%   -0.04%     
==========================================
  Files          78       78              
  Lines        6516     6516              
==========================================
- Hits         5339     5337       -2     
- Misses        832      833       +1     
- Partials      345      346       +1     

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[gemini-code-assist]

Code Review

This pull request introduces support for 'proto-only' Python APIs, which are APIs that do not have a GAPIC client. The changes are well-structured and include comprehensive tests for the new functionality and error handling. A detailed security audit found no high-severity or critical vulnerabilities, specifically noting safe handling of command execution with exec.CommandContext for protoc and python3, secure path construction for .proto files using filepath.Join and filepath.Base, and robust data handling. My feedback includes a minor suggestion to improve error wrapping for better diagnostics.

[jskeet]

@JoeWang1127 I can separate this out into the generation code and then the migration code if necessary, but I thought it made sense to do all in one go.