Discovery: Refactor and add Avahi DBus backend (#1347)

* 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

COMPILING.md

@@ -56,6 +56,17 @@ On Fedora systems:
 sudo dnf install alsa-lib-devel
 ```
 
+### Zeroconf library dependencies
+Depending on the chosen backend, specific development libraries are required.
+
+*_Note this is an non-exhaustive list, open a PR to add to it!_*
+
+| Zeroconf backend   | Debian/Ubuntu                | Fedora                            | macOS       |
+|--------------------|------------------------------|-----------------------------------|-------------|
+|avahi               |                              |                                   |             |
+|dns_sd              | `libavahi-compat-libdnssd-dev pkg-config` | `avahi-compat-libdns_sd-devel` |   |
+|libmdns (default)   |                              |                                   |             |
+
 ### Getting the Source
 
 The recommended method is to first fork the repo, so that you have a copy that you have read/write access to. After that, it’s a simple case of cloning your fork.