Refactor state machine of the signer

[jpraynaud]

Why

We need to update the state machine of the signer as a preparation phase of the decentralization of the signature orchestration.
We also need to remove the Signed state which adds latency to the signature of the signers by adding an extra cycle before the signer is able to sign.

What

Refactor the state machine of the signer with these new states and transitions:

• Unregistered -> RegisteredNotAbleToSign when registration succeeds but the signer can not sign the current epoch
• Unregistered -> ReadyToSign when registration succeeds and the signer can sign the current epoch
• ReadyToSign -> ReadyToSign when trying to create a new message if any and sign it

How

• can_signer_sign computation is done once per epoch (in EpochService?)
• Refactor the state machine of the signer with the new states and transitions
• Adapt integration tests
• Update documentation with new state machine diagram