[FEATURE] Add a Metric to Check if a Pool Owner is Registered with a DRep

[rayanebel]

Description

After the Plomin Hard Fork, all stakeholders must be registered with a DRep to withdraw their rewards otherwise, they will receive a tx error.

The goal is to create a new metric called cardano_validator_watcher_pool_registered_to_drep. This metric will monitor and indicate whether a stake pool owner is registered with a DRep.