Configuration options for RL-based CC and GCC on AlphaRTC

[yooneo0124]

• GCC inside AlphaRTC as a compilable branch #96
• Allow users to specify whether to run GCC or RL-based CC testing on AlphaRTC.
  • Create a boolean option gcc inside receiver.json and sender.json config
  • Create a single CC class that uses the config to decide whether to use GCC's or RL-based CC's estimated bandwidth for bitrate control.

Closes #97

[yooneo0124]

Closing this temporarily as #103 needs to be merged first.