Install action runners as Python packages, utilize dynamic runner loading and enumeration

A bunch of changes have been made so action runners are now fully blown Python packages.

This means they can be installed using pip (same thing as we do with auth backends).

We should make the following updates to utilize this new functionality:

• Build Python packages (wheels) for runners
• Update cicd and other workflows and scripts to install them as Python package in the same manner was we install auth backends
• Update runner registrar code to utilize dynamic enumeration of available / installed runners (right now it does on disk based introspection, but we should use stevedore "available drivers" mechanism)
• Document the changes, document how to install custom runner using pip from PyPi / git (e.g. CloudSlang one)