Refactor Security Mode Command procedure: send in RRC DL transfer
Previously, the CU sent the Security Mode Command as part of a UE context setup request. This was done "because it was possible", not because there was an inherent need to do this. However the LiteOn DU does not like this, as it expects to also have a DRB in the UE context setup request procedure, which is not always the case. Hence, send the Security Mode Command in a normal DL RRC msg transfer over F1. As of this commit, there is not UE context Setup Request (so it might not work with all DUs), but the OAI DU is cool and does not care, so RFsim still works. This also aligns the CU's behavior with O-RAN.WG5.C.1-v11. Finally, as of this commit, we do not trigger a UE context setup request, so we cannot handle PDU sessions inside the initial UE context setup request at the same time as the security mode command (which was done previously before reaching this point). This will be fixed in a later commit.
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