Minimal mdns: move responders into allocator class. (#7528)

* Minimal mdns: move responders into allocator class.

The code is basically the same, but moved into a subclass. This
is done for 2 reasons:
1) Makes the allocator class more testable (tests added)
2) Right now the commissionable and operational records are mixed
   in a single query responder. This is not ideal and we need to
   separate these into different query responders in a single
   ResponseSender. This will let us allocate a query responder for
   each record easily and separate all the record and qname
   allocations so they can easily be cleared. (upcoming PR)

* Restyled by gn

Co-authored-by: Restyled.io <[email protected]>