Add padding support to TrackLocalStaticSample

[alexpokotilo]

To add padding-only samples call GeneratePadding

Description

Sometime for Bandwidth estimation purposes we need to add Padding-only packages to the stream before switching to new rendition. This is TrackLocalStaticSample part of activity started in #251

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 77.51%. Comparing base (eb6e395) to head (c106615).
Report is 4 commits behind head on v3.

Files	Patch %	Lines
track_local_static.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #2745      +/-   ##
==========================================
+ Coverage   76.32%   77.51%   +1.18%     
==========================================
  Files          87       87              
  Lines        9846     8111    -1735     
==========================================
- Hits         7515     6287    -1228     
+ Misses       1866     1359     -507     
  Partials      465      465              

Flag	Coverage Δ
go	79.00% <0.00%> (+1.14%)	⬆️
wasm	65.09% <ø> (+0.64%)	⬆️

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