ECN support

[penhauer]

Just updated ecn's type from uint8 to rtcp.ECN plus actually reading the ECN value from the interceptor attributes.

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.34%. Comparing base (51d58cb) to head (7f6364b).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/rfc8888/interceptor.go	28.57%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
+ Coverage   78.67%   79.34%   +0.67%     
==========================================
  Files          82       82              
  Lines        5106     4067    -1039     
==========================================
- Hits         4017     3227     -790     
+ Misses        919      667     -252     
- Partials      170      173       +3     

Flag	Coverage Δ
go	79.34% <54.54%> (+0.67%)	⬆️
wasm	?

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