ref(feedback): extract utils and shim code from create_feedback

[aliu39]

This PR moves code from create_feedback.py to new files in the feedback module. This makes the file more manageable, and also allows imports between create_feedback (feedback consumer code) and ingest/userreport.py (the entrypoint for ingesting UR's/legacy feedback). See https://develop.sentry.dev/application-architecture/feedback-architecture/ for the full ingest pipeline.

Right now importing save_userreport from create_feedback.py leads to a circular import. We need this for incoming work to link new feedbacks from issue details. Extracting shim_to_feedback and making a shared lib/ will fix this problem, and help w/future maintenance. We shouldn't have as much dependence on create_feedback.py as we do now.

Details of changes:
Moved from feedback.usecases.create_feedback to feedback.lib.utils:

• FeedbackCreationSource enum
• UNREAL_FEEDBACK_UNATTENDED_MESSAGE constant
• is_in_feedback_denylist util

Moved from feedback.usecases.create_feedback to feedback.usecases.shim_to_feedback:

• shim_to_feedback, helper for porting legacy format to the new one

Corresponding tests are also moved.

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 3 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/feedback/usecases/shim_to_feedback.py	92.30%	3 Missing ⚠️

Additional details and impacted files

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- Hits       491802   489815    -1987     
- Misses      83019    83071      +52     
  Partials      743      743