Add attestation validation to L1 contract

[just-mitch]

To work towards SPRTN we will need to validate attestations on the blocks processed.

We will accept a block if it have signatures from >2/3 of the epoch committee.

As we for SPRTN will be using a simplified model of Proof-of-Governance to select the validator set.

This will not focus on the exact method of sampling for now, but use a simple method, and then address it later in #7603. For every epoch we will keep the samples set in storage, this is is really expensive! But it is not meant to be the most efficient, it is meant to provide a reference for the logic, not the most effective way to get to the result.

Checking the signatures itself is relatively simple, we will just be using the ol' reliable ECDSA signatures such that we can use ecrecover directly.

Notice, that as there are no tests currently that is expecting multiple potential sequencers etc, we should allow a case of "base race" if there are no validators in the set. Current E2E will then run this setup.