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

.github/workflows/test.yml

@@ -109,7 +109,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
         experimental: [false]
         # Ignore failures in beta
@@ -164,7 +164,7 @@ jobs:
       matrix:
         os: [windows-latest]
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
     steps:
       - name: Checkout code
@@ -215,7 +215,7 @@ jobs:
           - aarch64-unknown-linux-gnu
           - riscv64gc-unknown-linux-gnu
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
     steps:
       - name: Checkout code