Closes#5846
Will be moved down to the IPC service eventually.
The goal for connection roaming is not for totally transparent "Change
Wi-Fi networks without dropping SSH" handoffs, but just for Firezone to
re-connect itself as quickly as possible so that everything above us can
re-connect as quickly as it times out, and won't be hung up with a
broken tunnel.
Note that for GUI Clients, listening is still done by the GUI process,
not the IPC service.
Yak shave towards #5846. This allows for faster dev cycles since I won't
have to compile all the GUI stuff.
Some changes in here were extracted from other draft PRs.
Changes:
- Remove `thiserror` that was never matched on
- Don't return the DNS resolvers from the notifier directly, just send a
notification and allow the caller to check the resolvers itself if
needed
- Rename `DnsListener` to `DnsNotifier`
- Rename `Worker` to `NetworkNotifier`
- remove `unwrap_or_default` when getting resolvers. I don't know why
it's there, if there's a good reason then it should be handled inside
the function, not in the caller
```[tasklist]
### Tasks
- [x] Rename `*Listener` to `*Notifier`
- [x] (not needed) ~~Support `/etc/resolv.conf` DNS control method too?~~
```
