dnsdist: Add option to spoofRawAction to spoof multiple answers

[tjikkun]

Short description

This adds the option to specify multiple responses with spoofRawAction

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rgacogne]

Nice work! A few nits but nothing that would prevent a merge as it is.