feat: Add httpcore based HTTP2Transport

[BYK]

All our ingest endpoints support HTTP/2 and some even HTTP/3 which are significantly more efficient compared to HTTP/1.1 with multiplexing and, header compression, connection reuse and 0-RTT TLS.

This patch adds an experimental HTTP2Transport with the help of httpcore library. It makes minimal changes to the original HTTPTransport that said with httpcore we should be able to implement asyncio support easily and remove the worker logic (see #2824).

This should also open the door for future HTTP/3 support (see encode/httpx#275).

[codecov]

Codecov Report

Attention: Patch coverage is 84.95575% with 17 lines in your changes missing coverage. Please review.

Project coverage is 84.33%. Comparing base (55d757a) to head (b292dc5).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/transport.py	84.82%	11 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3588      +/-   ##
==========================================
- Coverage   84.35%   84.33%   -0.03%     
==========================================
  Files         133      133              
  Lines       13929    13994      +65     
  Branches     2937     2951      +14     
==========================================
+ Hits        11750    11802      +52     
- Misses       1442     1450       +8     
- Partials      737      742       +5     

Files with missing lines	Coverage Δ
sentry_sdk/client.py	83.49% <100.00%> (ø)
sentry_sdk/consts.py	99.50% <ø> (ø)
sentry_sdk/transport.py	85.19% <84.82%> (-0.13%)	⬇️

... and 1 file with indirect coverage changes

[sentrivana]

🥔 Thanks for this. Looks good, left some small comments, please take a look.

[sentrivana]

Merging this as soon as CI allows. :)