Add IPv4Packets templates

Further adaptation.

Added template for TC_DSLITE_B4_FRAG_BV_002.

Rename Dhcpv4Msg into DHCPv4Msg (aligned with DCHPv6Msg)

Extracted DHCPv6 definitions to be re-used in multiple sub folders.

And file comment on DSLite ATS

Re-ogranise DNS templates

Re-ogranise DNS templates

Check variant in getter functions.

Implemented getter functions.