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 84.90%. Comparing base (bd94010) to head (fe27100).
Report is 15 commits behind head on potel-base.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/transport.py	87.50%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           potel-base    #4572    +/-   ##
============================================
  Coverage       84.89%   84.90%            
============================================
  Files             144      158    +14     
  Lines           15012    15373   +361     
  Branches         2376     2423    +47     
============================================
+ Hits            12745    13053   +308     
- Misses           1539     1573    +34     
- Partials          728      747    +19     

Files with missing lines	Coverage Δ
sentry_sdk/client.py	84.88% <100.00%> (ø)
sentry_sdk/transport.py	84.13% <87.50%> (+1.08%)	⬆️

... and 21 files with indirect coverage changes