Refactor F1 UE context messages, add unit tests
Add encoder+decoder+util functions for F1 UE context messages, and use
them in the stack. The overall goal is to unit test encoder+decoder (to
be sure that what we send is what we receive), and remove any memory
leaks related to F1 handling. Messages that have a unit test:
- F1 UE context setup request/response
- F1 UE context modification request/response
- F1 UE context release request/command/complete
This MR removes almost all remaining memory leaks that are observable in
a "standard" configuration, running SISO in RFsim. In RFsim (without
channel modelling), there are only two memory leaks shown in asan, one
in L1, and one in NGAP.