Dump dbus subscription info with agent state

Add the list of current dbus subscriptions to the blob of data written to the logs when handling SIGUSR1. The blob now looks like this:

```
Apr 02 23:58:32 core-01 fleet[15001]: {"Agent":{"Systemd":{"DBUSSubscriptions":["ping.service","pong.service"]},"State":{"Offers":{},"Conflicts":{"ping.service":[],"pong.service":[]},"Bids":["pong.service"],"Peers":{"ping.service":["pong.service"]}}}}
```