Remove Aggregatorhash from Header and add aggregator set

[nashqueue]

In rollkit.proto we have

// Hash of block aggregator set, at a time of block creation
	bytes aggregators_hash = 12;

Replace it with:

// Block aggregator set, at a time of block creation
	repeated bytes signatures = 12;

[nashqueue]

Quotes from #788 (comment)

@nashqueue

It still feels that we might need to put the sequencer set in the header as this is something the proposer can set up arbitrarily, making the signatures useless before the fraud-proof window is closed, as setting a wrong validator set could be fraud itself. This is fine for now but will need discussion as it might be security relevant. I would propose keeping #784 open until we resolve this question.

@gupadhyaya

if the sequencer alters the signer keys before gossip, it can be considered fraud, which is not different from the sequencer altering the signature. both will be caught by the light node which fails to verify the signatures? i am not seeing the point of what is special with signing key altering fraud?

[Manav-Aggarwal]

Since the vA release for Rollkit will enforce centralized sequencers and support for multi-sequencer schemes will only be relevant post v1 release, this issue should block until then. See #1291