encvalue/decvalue/setencode: extend to accept PER ALIGNED/UNALIGNED/CANNONICAL mode

Summary

Currently the encvalue/decvalue/setencode accepts only "PER" unaligned encoding. Extend them to accept PER ALIGNED/UNALIGNED/CANNONICAL like: decvalue(bs, v_ngap_pdu,””,"PER:ALIGNED"); self.setencode(NGAP_PDU,"PER:ALIGNED")

/cc @aknappqwt @mmagyari