feat: Access gzip header through context

[effigies]

Closes #51.

[codecov]

Codecov Report

❌ Patch coverage is 76.59574% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.56%. Comparing base (e85ad33) to head (45b19e3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
- Coverage   90.89%   90.56%   -0.34%     
==========================================
  Files          13       13              
  Lines         846      890      +44     
  Branches      124      130       +6     
==========================================
+ Hits          769      806      +37     
- Misses         47       50       +3     
- Partials       30       34       +4     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ubdbra001]

This all makes sense to me.
mypy is complaining about 2 lines, but I think they are false positives. Might still be worth ignoring those rules for those lines so the errors don't crop up again in future.