Mithril Client validates a multi signature

[jpraynaud]

The Mithril Client must be able to validate a multi signature before restoring a snapshot with these assertions:

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

1. Verify a multi signature

• Compute the digest and verify a received multi signature with a Verifier trait and its implementation (and config & DI)

2. Download a certificate and restore snapshot

• Update the AggregatorHandler and its implementation to download the certificate from the aggregator (/certificate/{digest})
• Verify the certificate before triggering a restoration of the snapshot