Store delegators in pool state

[teodanciu]

Description

Store, in each pool's StakePoolState, the stake credentials that have delegated to that pool.
This set is updated in DELEG rule, for both shelley and conway:

• on (re)delegation
• when the stake credential is unregistered

The set is used to improve the performance of removing delegations when retiring the pool.
Based on PR: #5330

Closes: #4990

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
• CHANGELOG.md files updated for packages with externally visible changes.
  NOTE: New section is never added with the code changes. (See RELEASING.md).
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary.
  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.