onion-services: don't propagate restart of onion-addresses to depending services

Currently, public services that publish their onion addresses have a
`requires` dependeny on service `onion-addresses`, so they are restarted
when `onion-addresses` restarts.

This has the downside that most of nix-bitcoin's services are restarted
when a new onion service is added and the system config has the following common settings:
- nix-bitcoin.onionServices.bitcoind.public = true
- nix-bitcoin.operator.enable = true

Sequence of events:
1. onion service is added
2. `onion-addresses` restarts, because the new onion service is made available to `operator`
3. bitcoind restarts
4. all depending services restart

Fix this by using a `wants` dependency.

modules/onion-services.nix

@@ -80,7 +80,9 @@ in {
       systemd.services = let
         onionAddresses = [ "onion-addresses.service" ];
       in genAttrs publicServices (service: {
-        requires = onionAddresses;
+        # TODO-EXTERNAL: Instead of `wants`, use a future systemd dependency type
+        # that propagates initial start failures but no restarts
+        wants = onionAddresses;
         after = onionAddresses;
       });
     })