This commit implements adding a new UE on the target DU in case of
handover. We identify this case by the fact that the gNB-CU did not send
a gNB-DU UE ID (which is optional in F1AP, but not in our
implementation), such that we know there is no corresponding UE, which
therefore has to be created.

If it happens that there is a handover preparation information, we read
the UE capabilities from this information, and subsequently trigger the
creation of a new UE.

Another change is that for all bearers that we are asked to setup, we
add reestablishRLC so that the UE does a reestablishment during the
reconfiguration.

As we have to remove the reconfigurationWithSync and reestablishRLC IE
after application of the CellGroupConfig (because we might reuse the
CellGroupConfig later), free these IEs. As there are now multiple fields
to free, introduce a separate nr_mac_clean_cellgroup() function that
handles cleaning of the CellGroupConfig.
Co-authored-by: batuhanduyuler <batuhan.duyuler@firecell.io>
Co-authored-by: rmagueta <rmagueta@allbesmart.pt>