Correct F1 RRC Message Transfer

Follow-up after !1551 (merged) to clean up and handle DL/UL RRC Message Transfers more uniformly

Changes:

• to simplify the API of nr_rlc, I added nr_rlc_add_drb() and nr_rlc_add_srb() instead of nr_rrc_rlc_config_asn1_req()
• cleanup RRC: remove useless node switches to transfer DL RRC Messages uniformly (PDCP handles F1 and direct calls automatically)
• Create and store Cell group of new UE at DU. Create both SRB0 and SRB1 before sending Initial UL RRC Message transfer
• Do not decode the UE RRC Container in DL RRC Messages: they are transparent for the DU and not supposed to be decoded (?) -- at least this is my interpretation. We did this to check for the cellGroup and then set up the SRB1 with correct data from the RRC, when in fact, the cell group is created at the DU before and sent to the CU. S
• Do not handle DL RRC Messages at the RRC: the RRC sends these messages. We handled them in the DU's RRC, which is not necessary nor useful.