http3: add http3.Server.ServeQUICConn to serve a single QUIC connection

[marten-seemann]

This is needed when having a QUIC listener that accepts multiple ALPNs. If the HTTP/3 ALPN is negotiated, one would pass the connection to http3.Server.ServerQUICConn. If a different ALPN is negotiated, the connection would be passed to a different connection handler.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.49% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (7d6b0fc) compared to base (424a663).
Patch coverage: 53.85% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3579      +/-   ##
==========================================
- Coverage   85.50%   85.49%   -0.02%     
==========================================
  Files         139      139              
  Lines       10244    10252       +8     
==========================================
+ Hits         8759     8764       +5     
- Misses       1103     1105       +2     
- Partials      382      383       +1     

Impacted Files	Coverage Δ
http3/server.go	74.01% <53.85%> (-0.29%)	⬇️

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