[FEATURE] Add possibility to explicitly request address family on DNS lookups

[igorolhovskiy]

Is your feature request related to a problem? Please describe.

I'm having a dual-stack (IPv4/IPv6) system. For testing and maintenance reasons I need to select which address family I need to use in every particular request.

Describe the solution you'd like

One of the possible solutions - on forcing outbound socket to a specific address family, DNS request results are adopted to address family of the outbound socket.

Some core function like dns_query() that returns all IP addresses resolved might be helpful.

Other option might be in string transformation of {ip.resolve} where the family of address might be specified.

For the last 2 options, I'm not sure if setting $du to ipv4 (for example) address will fill all the headers (like Via) in a correct format, plus, provide seamless IPv4/v6 bridge.

Implementation

Not sure at all, but I guess, resolve.c in core

Describe alternatives you've considered

None for the moment.

Additional context

I need an ability to change protocol and address family during one call, like in one call with re-INVITE change proto and address family (for testing reasons)

[igorolhovskiy]

I've made some experiments, seems just setting $du to v4 address family works flawlessly
Added quick'n'dirty resolve4 and resolve6 transformations to test.
igorolhovskiy@527d8d3