fix(httpx): Prevent Sentry baggage duplication

[szokeasaurusrex]

Sentry baggage will get added to an HTTPX request multiple times if the same request is repeated. To prevent this from occurring, we can strip any existing Sentry baggage before adding Sentry baggage to the request.

Fixes #3709

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.89%. Comparing base (e9ec6c1) to head (3181f46).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/integrations/httpx.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3728      +/-   ##
==========================================
- Coverage   79.89%   79.89%   -0.01%     
==========================================
  Files         137      137              
  Lines       15373    15382       +9     
  Branches     2609     2610       +1     
==========================================
+ Hits        12282    12289       +7     
- Misses       2221     2222       +1     
- Partials      870      871       +1     

Files with missing lines	Coverage Δ
sentry_sdk/tracing_utils.py	84.03% <100.00%> (+0.13%)	⬆️
sentry_sdk/integrations/httpx.py	92.10% <88.88%> (-0.76%)	⬇️

... and 1 file with indirect coverage changes

[sentrivana]

Found a typo. Overall looks good to me. Can we also add an httpx test specifically for the scenario where we were accumulating baggage?