Shared datatypes and traits used by Mithril rust projects

Provide:

• Helpers for the Mithril STM lib with the [crypto_helper].
• [certificate chain][certificate_chain] used to validate the Certificate Chain created by an aggregator
• The [entities] used by, and exchanged between, the aggregator, signers and client.

Mithril-common

This is a work in progress 🛠

This crate contains common utility types and testing infrastructure used in the implementation of the Mithril nodes.