Easy configuration of RL-based CC or GCC evaluation and update the README

[jeongyooneo]

• In a main branch, 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.