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

Attention: Patch coverage is 0% with 110 lines in your changes missing coverage. Please review.

Project coverage is 69.49%. Comparing base (c06f448) to head (0674db3).

Files with missing lines	Patch %	Lines
pkg/pacing/interceptor.go	0.00%	71 Missing ⚠️
pkg/pacing/leaky_bucket_pacer.go	0.00%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   71.38%   69.49%   -1.89%     
==========================================
  Files          79       81       +2     
  Lines        4526     4636     +110     
==========================================
- Hits         3231     3222       -9     
- Misses       1158     1274     +116     
- Partials      137      140       +3     

Flag	Coverage Δ
go	69.49% <0.00%> (-1.76%)	⬇️
wasm	67.70% <0.00%> (-1.78%)	⬇️

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