Refactor signable builder services to compute full protocol message in signer/aggregator

[jpraynaud]

Why

We have some duplicated code when the signature of the aggregate verification key is appended to the signed protocol message. Also this operation is done in the runners of the nodes whereas it should be the responsibility of the signable builder services in the signer and aggregator.

What

Make the signable builder compute the full protocol message on its own in the signer and aggregator

How

• Refactor the signable builder service to compute the full protocol message on its own in the aggregator
• Refactor the signable builder service to compute the full protocol message on its own in the signer