F1AP: move GTP from enc/dec to logic handlers

The F1AP message encoders and decoders should do only that, encoding and decoding messages. For the actual logic handling, including set up and teardown of bearers, we should use the handlers.

This branch moves the GTP tunnel handling to the DU handler (in mac_rrc_dl_handler.c). in E1, the handler in cucp_cuup_handler.c does that already. Further, move GTP initialization to F1 initialization, instead of the first UE message.

TBD:

• handle QoS info in DU_handle_UE_CONTEXT_SETUP_REQUEST()