Use "local" SD in Mithril Signer

[jpraynaud]

The stake distribution must be retrieved from a local store that will be fed with data from a Cardano node (feeding will be done separately):

• Move store/adapter from mithril-aggregator to mithril-common
• Move 'stake_storefrommithril-aggregatortomithril-common`
• Use 'stake_store` in the signer