* discovery: use opaque error type for DnsSdError
This helps to decouple discovery and core by not leaking implementation
details of the zeroconf backend into Error conversion impls in core.
* discovery: map all MDNS/DNS-SD errors to DiscoveryError::DnsSdError
previously, libmdns errors would use a generic conversion
from std::io::Error to core::Error
* discovery: use an opaque type for the handle to the DNS-SD service
* discovery: make features additive
i.e. add with-libmdns instead of using not(with-dns-sd).
The logic is such that enabling with-dns-sd in addition to the default
with-libmdns will still end up using dns-sd, as before.
If only with-libmdns is enabled, that will be the default.
If none of the features is enabled, attempting to build a `Discovery`
will yield an error.
* discovery: add --zeroconf-backend CLI flag
* discovery: Add minimal Avahi zeroconf backend
* bump MSRV to 1.75
required by zbus >= 4
* discovery: ensure that server and dns-sd backend shutdown gracefully
Previously, on drop the the shutdown_tx/close_tx, it wasn't guaranteed
the corresponding tasks would continue to be polled until they actually
completed their shutdown.
Since dns_sd::Service is not Send and non-async, and because libmdns is
non-async, put them on their own threads.
* discovery: use a shared channel for server and zeroconf status messages
* discovery: add Avahi reconnection logic
This deals gracefully with the case where the Avahi daemon is restarted
or not running initially.
* discovery: allow running when compiled without zeroconf backend...
...but exit with an error if there's no way to authenticate
* better error messages for invalid options with no short flag
