Add ability to set http headers in client

[EthanHeilman]

This adds the ability to specify additional HTTP headers in the Extended Connect.

cl := masque.Client{
  QUICConfig:      quicConf,
  TLSClientConfig: tlsConf,
  Headers:         http.Header{"EXAMPLE_HEADER": []string{"example_header_value}},
}
tunnelConn, _, err := cl.Dial(
  context.Background(),
  uritemplate.MustNew(template),
  targetAddr,
)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.41%. Comparing base (b312bc5) to head (1ca9b66).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   74.08%   74.41%   +0.33%     
==========================================
  Files           4        4              
  Lines         382      387       +5     
==========================================
+ Hits          283      288       +5     
  Misses         77       77              
  Partials       22       22              

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[marten-seemann]

Which issue does this resolve?

[EthanHeilman]

@marten-seemann The issue this solves is that I wanted to set HTTP headers in the extended connect and AFAIC masque-go didn't expose a way to do this.

I was writing some code, ran into some features I needed in masque-go. Added them to my local fork and then figured I'd push them upstream in case upstream seems them as valuable. I have one more PR incoming.

[marten-seemann]

@marten-seemann The issue this solves is that I wanted to set HTTP headers in the extended connect

I can see that :)
My question is: what do you need this for?

[EthanHeilman]

I'm doing reverse QUIC tunnels for rendezvous nodes. I'm using the HTTP headers to offer the ability to associate arbitrary metadata with the tunnel on first connect.

I don't think this use case is that unusual HERMES does something similar with HTTP headers in QUIC proxies.