Implement pacing interceptor

[mengelbart]

Description

Implements a pacing interceptor separate from congestion control. The interceptor calls back to implementations of the Pacer interface whenever a packet is sent.

[codecov]

Codecov Report

❌ Patch coverage is 71.20000% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.62%. Comparing base (8492094) to head (808eb87).

Files with missing lines	Patch %	Lines
pkg/pacing/interceptor.go	78.07%	20 Missing and 5 partials ⚠️
pkg/pacing/rate_limit_pacer.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   78.62%   78.62%   -0.01%     
==========================================
  Files          82       84       +2     
  Lines        5109     5234     +125     
==========================================
+ Hits         4017     4115      +98     
- Misses        921      946      +25     
- Partials      171      173       +2     

Flag	Coverage Δ
go	78.62% <71.20%> (-0.01%)	⬇️

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