software/ors-amarisoft: gnb: Move DRB configuration to standalone file

A preparatory step for multiRU for the same reason as for LTE and also to be
able to support Carrier Aggregation in between NR and LTE cells in the future
(called Dual Connectivity by 3GPP).

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu
/reviewed-on !1473