feat: Add concurrent.futures integration

[alexander-alderman-webb]

Automatically fork isolation and current scopes when running tasks with concurrent.future. The implementation is essentially packaging work-around #4508 (comment) as an integration.

The tests are adapted from test_threading.py, but only use ThreadPoolExecutor.

Closes #4565

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.58%. Comparing base (dcefe38) to head (431bcf5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4770      +/-   ##
==========================================
+ Coverage   84.56%   84.58%   +0.01%     
==========================================
  Files         158      159       +1     
  Lines       16486    16513      +27     
  Branches     2856     2858       +2     
==========================================
+ Hits        13942    13968      +26     
- Misses       1697     1698       +1     
  Partials      847      847              

Files with missing lines	Coverage Δ
sentry_sdk/integrations/concurrent.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes