[ffx][lib][mdns_discovery]: Add convenience methods

Adds convenience method `discover_targets` which will listen for a given
duration and return the Vec of all targets discovered via mDNS during
that interval.

Change-Id: I601d6ea6ed280769164f27bf155d1493bdb4c53a
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/809933
Commit-Queue: Colin Nelson <colnnelson@google.com>
Reviewed-by: Megan Batty <mgnb@google.com>