F1AP DU: Init GTP on UEContextSetupRequest
This MR addresses the following: For the current implementation, in the DU, the GTP instance for F1-U is only initialized on UEContextModificationRequest. In cases for UE-reattachment, for instance, after the DU is restarted, the DRB setup may come in the UEContextSetupRequest instead of UEContextModificationRequest. This causes the user plane to not work in such cases since there are no existing GTP instances to create GTP tunnels at F1-U.
This MR is a follow-up for !2578 (closed). The proposed fix is adapted from the existing code from UEContextModificationRequest. In the long term, perhaps some refactoring work is needed, and the GTP instance initialization should be done when the DU is started?