added rest api endpoint to fetch closest available slot

[rolfyone]

Added /teku/v1/beacon/state/finalized/slot/before/{slot}

This endpoint will return the last stored finalized state before (inclusive) the specified slot.

This should improve the current situation of archive node being slow, as at least you can compute how many states need to be rebuilt.

While this does provide helpful information currently, the better fix is to fix the archive storage.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.