feat(sessions): Add top-level start- and end session methods

[szokeasaurusrex]

Closes #4473.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.59%. Comparing base (0e21fa2) to head (e897304).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4474   +/-   ##
=======================================
  Coverage   80.59%   80.59%           
=======================================
  Files         142      142           
  Lines       16030    16036    +6     
  Branches     2745     2745           
=======================================
+ Hits        12920    12925    +5     
- Misses       2243     2244    +1     
  Partials      867      867           

Files with missing lines	Coverage Δ
sentry_sdk/__init__.py	100.00% <ø> (ø)
sentry_sdk/api.py	80.40% <100.00%> (+0.82%)	⬆️

... and 3 files with indirect coverage changes