In this commit, handle the reception of a UE context setup response, in
the case of handover. In that case, store the gNB-DU UE ID and C-RNTI of
the UE at the target DU, and trigger a reconfiguration towards the UE at
the source DU via the callback.

To ensure that the CU-UP's PDCP is SN-aligned with the UE, use
cuup_notify_reestablishment() to reset the bearers with PDCP
reestablishment, which has already been implemented in the last commit.
Co-authored-by: batuhanduyuler <batuhan.duyuler@firecell.io>
Co-authored-by: rmagueta <rmagueta@allbesmart.pt>