mdns component does not support query for subtypes (IDFGH-15167)

[sv-abl]

Is your feature request related to a problem?

Neither current master (current release of mDNS component is v1.8.2 at the time of writing) nor the current state of the refactoring in PR #776 does support querying for subtypes as described in section 7.1 Selective Instance Enumeration (Subtypes) of RFC 6763: DNS-Based Service Discovery (https://www.rfc-editor.org/rfc/rfc6763.html#section-7.1).

Describe the solution you'd like.

New functions allowing to query / browse for a certain subtype only should be added ot the mdns component API.

Describe alternatives you've considered.

Alternatively, the existing API (signature of mdns_query, etc.) could be expanded by adding at least one additional argument to allow querying for a certain subtype if desired.

(Just for clarity: prefixing e.g. "_my-service-type" by some string like "mysubtype." and supplying that as service_type to, e.g., mdns_query is no possible workaround, as the subtype must become a distinct label in the mDNS packet, and additionally it may consist of arbitrary 8-bit data values, including \0)

Additional context.

Note that announcing subtypes is already supported, e.g., using mdns_service_subtype_add_for_host, although the current implementation has some design flaw as discussed in issue #804.