Refactor SCTP Association Response Handling and E1 connection loss at CU-UP

• Refactor SCTP Association Response Handling
  • Unified function for both CU-CP and CU-UP handling
  • CU-CP: Notifies RRC of E1 connection loss with CU-UP
  • CU-UP: Initiates a new SCTP association request to regain E1 connection
  • Renamed e1apHandleTimer() to e1ap_reconnect for clarity
  • this fix will prevent issue #878 from happening
• Send NGAP Setup Failure when No CU-UP is associated and still the CU-CP receives a PDU Session Setup Request
  • add pdu_session_setup_failure for handling NGAP Setup Failures
  • this is an extra failure handling to make #878 is handled if it ever happens again
• NGAP Cause Refactoring
  • Refactor struct for NGAP cause group and value
  • Add extended radio network cause enumeration values
  • Refactor redundant code into a new NGAP cause encoding function
• Minor logging, function descriptions improvements

Closes #878