feat(tracing): Add __repr__ to Baggage

[szokeasaurusrex]

The default __repr__ does not show what is in the Baggage, making it extremely difficult to debug code involving Baggage objects. Add a __repr__ which includes the serialized Baggage to improve debuggability.

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[szokeasaurusrex]

Gonna add a simple test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.25%. Comparing base (c227e11) to head (9115b96).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4043   +/-   ##
=======================================
  Coverage   80.24%   80.25%           
=======================================
  Files         139      139           
  Lines       15415    15417    +2     
  Branches     2606     2606           
=======================================
+ Hits        12370    12373    +3     
  Misses       2203     2203           
+ Partials      842      841    -1     

Files with missing lines	Coverage Δ
sentry_sdk/tracing_utils.py	84.51% <100.00%> (+0.08%)	⬆️

... and 1 file with indirect coverage changes