Support Extra DHCP Options for IPv4 and IPv6

Add API and DB change for Blueprint extra-dhcp-opts-ipv4-ipv6.
Add unit tests for this change.

The validation of input extr dhcp options is not included
in this commit. A follow-up commit will be added for
validation.

Change-Id: I85d039f4618ec0b530e1e8b89a05993885ffdcd9
Partially-implements: Blueprint extra-dhcp-opts-ipv4-ipv6