bridge: Implement a new dbus-json3 protocol payload

This protocol is much simpler than the others. There will be some
additions to it in the future, but at it's core it's about relaying
messages and signals.

We don't have innate object manager support in the bridge, the
javascript code will respond to events and update an object model
there.
base: Implement a javascript client for new dbus-json3

This implements a simple javascript client for dbus-json3. It has
no proxy support at this time, but that will be added later.
doc: Add documentation for cockpit.dbus and related

Closes #1356