add a multiplexer for the tracer

[marten-seemann]

This allows users to easily attach multiple tracers, by passing a Tracer constructed from logging.NewMultiplexedTracer() in the quic.Config.Tracer.

[codecov]

Codecov Report

Merging #2665 into master will increase coverage by 0.05%.
The diff coverage is 92.86%.

@@            Coverage Diff             @@
##           master    #2665      +/-   ##
==========================================
+ Coverage   86.68%   86.74%   +0.05%     
==========================================
  Files         123      124       +1     
  Lines        9778     9862      +84     
==========================================
+ Hits         8476     8554      +78     
- Misses        970      973       +3     
- Partials      332      335       +3     

Impacted Files	Coverage Δ
logging/multiplex.go	92.86% <92.86%> (ø)

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