WIP: simplify interface to TWCC 4/4

[jech]

This is part 4/4 of a cross-repository series.

Part 1 is pion/interceptor#297
Part 2 is #3002
Part 3 is pion/interceptor#298

Please don't merge until I've had a chance to test it in Galene.

@Sean-Der Please let me know if you agree with the principle, so I can start working on making Galene use this interface. Even if you approve, I won't merge until I've convinced myself that it works for Galene.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.68%. Comparing base (f2191fb) to head (ef9a579).
Report is 78 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (f2191fb) and HEAD (ef9a579). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (f2191fb)	HEAD (ef9a579)
go	2	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3003       +/-   ##
===========================================
- Coverage   77.77%   63.68%   -14.10%     
===========================================
  Files          89       67       -22     
  Lines       10578     3957     -6621     
===========================================
- Hits         8227     2520     -5707     
+ Misses       1857     1308      -549     
+ Partials      494      129      -365     

Flag	Coverage Δ
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wasm	63.68% <ø> (ø)

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