http3: expose ALPN values

[marten-seemann]

When using a custom quic.Listener to accept QUIC connections, it's useful to be able to set the correct ALPN.

Constant name chosen loosely according to https://pkg.go.dev/golang.org/x/net/http2#pkg-constants.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.50% // No change to project coverage 👍

Coverage data is based on head (452a6da) compared to base (424a663).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3580   +/-   ##
=======================================
  Coverage   85.50%   85.50%           
=======================================
  Files         139      139           
  Lines       10244    10244           
=======================================
  Hits         8759     8759           
  Misses       1103     1103           
  Partials      382      382           

Impacted Files	Coverage Δ
http3/server.go	74.29% <100.00%> (ø)

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