This method encodes ePDG Id so it can be stored to EF.ePDGId or EF.ePDGIdEm.
See 3GPP TS 31.102 version 13.4.0 Release 13, section 4.2.102 and 4.2.104.
Resulting hex string is made of tag value + length + address type + address.
tag value for home ePDG identifier is 80
address type: 0x00 (FQDN), 0x01 (IPv4), 0x02 (IPv6), other (Reserved)
ePDG FQDN example: epdg.mnc001.mcc001.3gppnetwork.org
Note: Only FQDN format is supported for now
Change-Id: I864bda5505e9061391a727add294a6e90c50f9ef