Add Store Protocol Parameters in Aggregator

[jpraynaud]

Issue

This store is needed in the Aggregator to keep track of the Protocol Parameters.

These Protocol Parameters are then stored in the Protocol Initializer Store of the Signer. Thus, it does not need a dedicated store for keeping track of these parameters.

Tasks

• A Protocol Parameters Store in the Aggregator
• Update configuration where needed
• Update documentation where needed
• Add Protocol Parameters as a configuration
• Store Protocol Parameters in runtime
• Use Protocol Parameters in multi signer
• Add next_protocol field in Pending Certificate