ref(transport): Add shared sync/async transport superclass and create a sync http subclass

[srothh]

Moved shared sync/async logic into superclass, and moved sync transport specific code into a new subclass(BaseSyncHttpTransport), from which the current transport implementations inherit. Note that currently the threaded worker is still being created in the superclass. In a next step, I want to add an abstract worker class for both the threaded and async task worker, however I wanted to keep this PR atomic and only re-implement the sync transport with the new hierarchy.

Fixes GH-4568

[codecov]

Codecov Report

Attention: Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.

Project coverage is 84.94%. Comparing base (bd94010) to head (19f7211).
Report is 7 commits behind head on potel-base.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/transport.py	87.80%	5 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4572      +/-   ##
==============================================
+ Coverage       84.89%   84.94%   +0.04%     
==============================================
  Files             144      158      +14     
  Lines           15012    15373     +361     
  Branches         2376     2423      +47     
==============================================
+ Hits            12745    13059     +314     
- Misses           1539     1568      +29     
- Partials          728      746      +18     

Files with missing lines	Coverage Δ
sentry_sdk/transport.py	84.13% <87.80%> (+1.08%)	⬆️

... and 20 files with indirect coverage changes