[MDNS] Remove all on-stack buffers from our built-in mDNS

[ivmarkov]

Turns out the domain library is versatile enough so that we can completely get rid of on-stack buffers like this one.

Ongoing work in this branch of edge-mdns.
If successful (which seems more and more likely now that I can serialize even TXT records without extra buffers), I'll merge back bits and pieces from this new implementation into rs-matter.