Draft: Add E2 agent

Adds FlexRIC's E2 Agent as a submodule with complete KPM v3 and RC 1.3 To enable in 5G, add --e2 on the command line. To configure in the configuration file, add the following block:

e2_agent = {
  near_ric_ip_addr = "127.0.0.1";
  sm_dir = "/path/where/the/SMs/are/located/"
}