http3: add support for HTTP Datagrams (RFC 9297)

[marten-seemann]

Fixes #3522. Closes #4444. Closes #4430. Depends on #4445 and #4460.

Doesn't rely on the OnStateChange stream API (#4428).

[codecov]

Codecov Report

Attention: Patch coverage is 91.59664% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 84.80%. Comparing base (11b1159) to head (f962f28).

Files	Patch %	Lines
http3/conn.go	87.10%	6 Missing and 6 partials ⚠️
http3/datagram.go	95.08%	2 Missing and 1 partial ⚠️
http3/http_stream.go	72.73%	3 Missing ⚠️
http3/client.go	90.48%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4452      +/-   ##
==========================================
+ Coverage   84.73%   84.80%   +0.07%     
==========================================
  Files         152      154       +2     
  Lines       14478    14645     +167     
==========================================
+ Hits        12267    12419     +152     
- Misses       1704     1714      +10     
- Partials      507      512       +5     

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