Add refactored GCC implementation

[mengelbart]

This PR contains a refactored version of the GCC implementation from pion/interceptor. It includes a simulation test that runs >100 simulations using vnet from pion/transport. Simulations are hidden behind a simulation build tag, because they generate a lot of log files. The simulation package contains a simple python script to create plots for each of the simulation experiments. In a future PR, I plan to replace the GCC implementation in pion/interceptor with the one implemented here.

To run the simulation, run go test -v -tags=simulation ./simulation/. It will create a directory simulation/logs containing two log files for each run, one containing packet logs from the packet dumper interceptor and one containing pion logs. To create plots, run ./main.py from within the simulation directory. The plots will be placed in the logs directory. Alternatively, you can pass input or output to the script manually, using the '-i', or '-o' flags.

This PR should not be merged before pion/interceptor#374. When that PR is merged, the go.mod reference in this PR needs to be updated.

[codecov]

Codecov Report

❌ Patch coverage is 54.95050% with 182 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.95%. Comparing base (e2f5773) to head (6fb5b52).

Files with missing lines	Patch %	Lines
gcc/delay_rate_controller.go	0.00%	55 Missing ⚠️
gcc/kalman.go	0.00%	45 Missing ⚠️
gcc/send_side_bwe.go	0.00%	43 Missing ⚠️
gcc/state.go	69.56%	14 Missing ⚠️
gcc/usage.go	0.00%	10 Missing ⚠️
gcc/arrival_group_accumulator.go	80.95%	7 Missing and 1 partial ⚠️
gcc/rate_controller.go	83.33%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #2       +/-   ##
===========================================
+ Coverage        0   54.95%   +54.95%     
===========================================
  Files           0       11       +11     
  Lines           0      404      +404     
===========================================
+ Hits            0      222      +222     
- Misses          0      181      +181     
- Partials        0        1        +1     

Flag	Coverage Δ
go	54.95% <54.95%> (?)

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