Reintroduce separate message types to simplify the encoding/decoding,
including shorter names. The old types will be removed once everything
has been migrated to the new encoder/decoder, which should ensure that
everything is properly migrated to the unit-tested encoder/decoders.
Also, for each type of F1 message (F1 UE context setup request,
response, modification request, ...) introduce a separate type to avoid
to mistakenly use wrong fields, which can lead to bugs. In this regard,
the DL PDCP SN bit size in the DRB-ToBeSetup list of the UE context
setup request is optional, as it will also be used in the UE context
modification request DRB SetupMod list (which avoids quite some code
duplication). The encoder checks that this IE is present, though.
rmagueta