add support for the Extended CONNECT method

[marten-seemann]

Extended CONNECT is used by WebTransport.

[codecov]

Codecov Report

Merging #3357 (89d7847) into master (d4293fb) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3357      +/-   ##
==========================================
+ Coverage   85.57%   85.59%   +0.02%     
==========================================
  Files         135      135              
  Lines        9860     9870      +10     
==========================================
+ Hits         8437     8448      +11     
+ Misses       1046     1045       -1     
  Partials      377      377              

Impacted Files	Coverage Δ
http3/request.go	93.75% <100.00%> (+1.16%)	⬆️
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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