Pluggable data layer: transition backend/metrics to use type aliases from datalayer package

[elevran]

This PR introduces several interfaces and wrappers to allow users of backend/metrics code to work with datalayer as an implementation.

• Redefined structures in backend and metrics (e.g., MetricsState, PodMetrics, Pod) as aliases to corresponding types in datalayer
• Introduced interfaces to decouple rest of system (e.g., datastore) from the concrete implementations of backend/metrics.
  For example, EndpointFactory interface variable in datastore instead of a concrete PodMetricsFactory struct.
• Added no-op methods to podMetrics so it implements the expected extended attribute interface (another option is to define Endpoint without it and type assert from Endpoint to (e.g.,) EndpointWithExtendedAttributes inside the datalayer).
• RefreshLoop handling is now delegated to the EndpointFactory which allows removing StopRefreshLoop from the public interface (the factory was already calling the private startRefreshLoop method).

In terms of next steps to complete the co-existence:

• Implement EndpointFactory for datalayer (e.g., manage Collectors for endpoints).
• Enable runner.go to create the right EndpointFactory type, based on environment variable (e.g., EXPERIMENTAL_DATALAYER_V2 set for datalayer package based implementation).
• minor tweaks in implementation (e.g., automatic data source registration, error logging to match existing cases such as undefined Pool checked on collection).

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[elevran]

/cc @ahg-g @kfswain

[ahg-g]

/lgtm
/approve

Thanks

[k8s-ci-robot]

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