Mithril Signer produces a single signature

[jpraynaud]

The Mithril Signer must be able to create a single signature of a digest with the following assertions:

• The real Mithril cryptographic library is used
• Stake distribution is static and fake
• Signer keys are static

1. Produce and send signatures

• Load a fake signer private keys on the Signer (use fake_data::signer_keys(party_id) in aggregator from Setup Mithril Network fake cryptographic artifacts #148)
• Retrieve at regular interval the pending certificate with a SignerScheduler trait and its implementation (and config & DI)
• If a new a beacon is detected and if its party_id is registered in the stake distribution of the pending certificate, it can compute the digest (by concatenating the Beacon parameters)
• Compute the single signature of the digest with a SingleSigner trait and its implementation (and config & DI)
• Send a single signature to the aggregator by upgrading the previously implemented AggregatorHander