Fix border condition in Snowbridge free consensus Updates (#5671)

# Description

A fix for a border condition introduced with new feature
https://github.com/paritytech/polkadot-sdk/pull/5201. A malicious
relayer could spam the Ethereum client with sync committee updates that
have already been imported for the period. This PR adds a storage item
to track the last imported sync committee period, so that subsequent
irrelevant updates are not free.

Original PR: https://github.com/Snowfork/polkadot-sdk/pull/172

## Integration

Downstream projects are not affected. Relayers will not be able to spam
the Ethereum client with irrelevant sync committee updates for free.

## Review Notes

Adds a storage item to track the last free sync committee update period,
so that duplicate imports are not free.

---------

Co-authored-by: Adrian Catangiu <[email protected]>