Add Stake Shares in Certificate

[jpraynaud]

Issue

It appears that we need to link the security level given by the protocol parameters to the share of Cardano stakes enrolled in Mithril network. If this is the case, we probably can't get these information from the current version of the certificate.

We could add 2 new fields to the metadata of the certificate:

• stakes/total_stakes_cardano in Lovelace
• stakes/total_stakes_mithril in Lovelace

:warning: Question: Do we need to embed these information in the message that is signed instead of the metadata? @iquerejeta @curiecrypt WDYT?

To do

• [ ] Compute these total stakes from the Stake Distribution already available in the Signer/Aggregator.
• [ ] Increase minor version of the Mithril API version
• [ ] Handle breaking change (when parsing entity) in the certificate chain with a chain re-genesis of all the Mithril networks

Relates to #513