Enhance aggregator support for signatures when not ready

[jpraynaud]

Why

The aggregator is not properly handling the reception of single signatures when it is not ready to sign (in particular when it is blocked). In that case, a long error message is logged which prevents their readability and exploitation during debugging.

What

Enhanced handling of incoming signatures when the aggregator is not ready.

How

• Add a new service SingleSignatureRegistrationGate in the aggregator
• Add a new get_status(&self)->SingleSignatureRegistrationStatus method and a SingleSignatureRegistrationStatus enum
• The gate is closed when the state machine is in the Blocked state
• The gate is opened during the Idle state
• The /register-signature route returns a 503 error if the gate is closed, and is not modified otherwise