watchtower: create a new cmd directory to enable a standalone watch tower

[Roasbeef]

As implemented today, a watch tower can only run in a "monolith" mode beside lnd. It should be possible to run a binary that only runs a watch tower, to allow for more flexible deployment configurations. Most of the scaffolding is already in place for towers, since they're able to run as a part of lnd. The remaining work is to "daemonize" the existing scaffolding such that the tower can run on its own. This stand-lone tower binary (towerd?) should come equipped with the current watch tower sub-server, and should also be able to interact with using the lncli tower command.

[orbitalturtle]

Going to take a stab at this.

[saubyk]

Converting this to a discussion. Can be reverted to the issue or a new one opened up if any development is planned for this.