[E2 agent] E2AP README update and OAI-FlexRIC CI pipeline improvements - E2AP README - add OSC nearRT-RIC section; update with proper FlexRIC links, and explanations on prerequisites, as well as for xApps - OAI-FlexRIC CI - use separate containers for xApps - add reference files for CU-UP/CU-CP - add logging for KPM NSSAI condition