Add delay rate controller

[mengelbart]

The delay rate controller updates the rate as described here: https://datatracker.ietf.org/doc/html/draft-ietf-rmcat-gcc-02#section-5.5

[codecov]

Codecov Report

❌ Patch coverage is 97.76536% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.06%. Comparing base (67fd82f) to head (c796323).

Files with missing lines	Patch %	Lines
gcc/delay_rate_controller.go	96.07%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main      #34      +/-   ##
===========================================
- Coverage   100.00%   99.06%   -0.94%     
===========================================
  Files            7       10       +3     
  Lines          251      430     +179     
===========================================
+ Hits           251      426     +175     
- Misses           0        2       +2     
- Partials         0        2       +2     

Flag	Coverage Δ
go	99.06% <97.76%> (-0.94%)	⬇️

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