Introduce new functions to add/release LCs without needing to go through
the intermediate CellGroupConfig. This harmonizes LC handling at the "DU
handler" to keep it consistent with RLC bearer handling.

These functions allow to group LC configuration handling, and we can
remove functions for setting NSSAIs and QoS by handling in one central
place (while adding LCs).

nr_mac_add_test_ue() is used by simulators and also in NSA mode to add a
new UE. Ideally, these users should also call the ue context setup
request handler to add a UE context; for the moment, respect the passed
list of LCs to add. Similarly, for do-ra, we require a dedicated
function to still parse the CellGroupConfig, so we keep that part of
functionality.

On reestablishment, since we do not process the CellGroupConfig from the
old UE, we need to add LCs for the new UE based on the onces from old
UE.