ext: add libmicrodns-based mdns device provider

The provider for now only detects and handles rtsp devices, but more protocols should be easy to add.

Marking as WIP until https://github.com/videolabs/libmicrodns/pull/24 is merged (it's about to be), for now this can be tested with gst-build and this wrap file in subprojects/libmicrodns.wrap:

[wrap-git]
directory=libmicrodns
url=git@github.com:MathieuDuponchelle/libmicrodns.git
revision=meson

If there are eg Axis cameras on the network, they should show up in the output of gst-device-monitor-1.0 -f