introduce Transport.ConnContext, use client's context on the connection

[marten-seemann]

For #4422.

TODO:

• add documentation for quic-go.net

This will allow us to remote the ConnectionTracingID.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 85.12%. Comparing base (e2fbf3c) to head (3df5170).

Files	Patch %	Lines
server.go	54.55%	4 Missing and 1 partial ⚠️
connection.go	66.67%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4507      +/-   ##
==========================================
- Coverage   85.15%   85.12%   -0.03%     
==========================================
  Files         154      154              
  Lines       14794    14803       +9     
==========================================
+ Hits        12597    12600       +3     
- Misses       1690     1694       +4     
- Partials      507      509       +2     

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[marten-seemann]

Added another test to the integration test: previously, the context on tls.Config.GetCertificate was not tested.