[BZ 1022943] Ability to trigger update of plugins on agents from UI and ...

...API

This is stage 1 of the series of commits to enable more user-friendly
handling of plugin updates.

It provides basic support for triggering updates of plugins on agents
by calling server-side APIs/GUI.

As a second stage, not yet implemented, I want to add the ability to track progress of plugin scans. This will then enable predictable behavior of plugin updates and the consequent pushes to the agents and/or other operations that might rely on the plugin scan being complete.

THIS IS NOT READY TO BE MERGED ATM, but I'm opening the PR to gather input on the addition of a new remote interface and other slightly invasive code changes.