[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

TODO:

• wait until mosaic5g/flexric!52 (merged) is merged
• remove sharedXappLibs from docker-compose.yml file