Add the watchtower client interface?

I'm interested in adding some watchtower client metrics to the https://github.com/lightninglabs/lndmon repository. Since they're using this repo for their lnd interface I'd like to add the watchtower client interface here first.

I was thinking I'd add a `wtclient_client.go` module in the same shape as the other lnd services. Is this something you guys are open to?