Refactoring listener app with configurable fallback

[nikola-jokic]

Listener app has been working well, but navigating through it is can be hard sometimes.

This PR aims to split the listener functionality into multiple testable packages:

• app: Actual listener app responsible for wiring listener, worker and metrics server together. Reads configuration and instantiates components.
• config: Shared package between the controller and the listener.
• listener: Package responsible for communicating with the actions service and calling worker methods when needed
• worker: Actual k8s worker doing patches on resources
• metrics: Package exposing metrics server and metrics publishers (interface used to translate actions service message to an actual metric)

This change does not include any new features. It is just a refactor of the original app. In case of an issue, users can set LISTENER_ENTRYPOINT to /github-runnerscaleset-listener in order to use older listener app.

NOTE: Even with the ability to fall back to the previous implementation, please submit issues related to the newer implementation so we can fix them and later have only one listener app.

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To be continued

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Awesome work @nikola-jokic <3 let's get this rolling! 🚀