chore(inactivity): Rename `inactive` flag to `sluggish`, add queries, events, and metrics

[gitferry]

The inactive flag of a finality provider is confused with the finality providers that are not in the active set. So, this PR is to rename the flag to sluggish, which is a common concept in the BFT world to describe validators who should vote but the votes are delayed for some reason thus committing sluggish faults.

This PR also made the following additions:

• queries for signing info
• events emitted for sluggish finaltiy providers being detected or reverted
• metrics for counting the number of sluggish finality providers
• docs about the new feature

[gitferry]

great work! Sluggish sounds a bit weird, but I agree inactive may be super confusing so 👍

Yeah, tbh it's werid to me, too, but it's just temporal and will be changed to jailed eventually