[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