bugfix: fix sdap

There was a problem testing accelleran cu + oai du + oai ue + rfsim because accelleran cu configures sdap header for UL but not for DL and our sdap implementation deals with either "no sdap header at all" or "sdap headers for both UL and DL".

The code has been modified to deal with the other cases ("sdap header for UL but not DL" and "sdap header for DL but not UL").

has_sdapDLheader and has_sdapULheader have been removed, has_sdap too, replaced by has_sdap_rx and has_sdap_tx, which makes the code independant of ue or gnb and is, I think, clearer.

With this work, accelleran cu + oai du + oai ue + rfsim works. We also have oai gnb + oai ue + rfsim functional for all cases of sdap header configuration (this won't happen for users, the code of gnb has to be modified to have sdap header for only ul or only dl). It also works with a COTS UE for all cases of sdap configuration (again, this won't happen for users, only "no sdap header" and "sdap headers for both UL and DL" exist in oai gnb with the current code).

To have other cases, you can edit openair2/RRC/NR/rrc_gNB_radio_bearers.c and search for NR_SDAP_Config__sdap_HeaderDL_present in there.

To enable sdap, run the gnb with the command line argument --gNBs.[0].enable_sdap 1