allow HTTP clients and servers to take over the request stream

[marten-seemann]

Depends on #3435.

[codecov]

Codecov Report

Merging #3437 (3eed9d1) into master (ccf897e) will increase coverage by 0.02%.
The diff coverage is 79.27%.

@@            Coverage Diff             @@
##           master    #3437      +/-   ##
==========================================
+ Coverage   85.69%   85.71%   +0.02%     
==========================================
  Files         135      136       +1     
  Lines        9946     9950       +4     
==========================================
+ Hits         8523     8528       +5     
+ Misses       1047     1046       -1     
  Partials      376      376              

Impacted Files	Coverage Δ
http3/body.go	77.78% <66.67%> (-17.78%)	⬇️
http3/response_writer.go	75.56% <66.67%> (+1.09%)	⬆️
http3/client.go	90.00% <70.37%> (-0.91%)	⬇️
http3/http_stream.go	92.86% <92.86%> (ø)
http3/request_writer.go	63.72% <100.00%> (-0.62%)	⬇️
http3/server.go	74.37% <100.00%> (+0.25%)	⬆️

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