Mithril signer in Amaru

## Why

We aim to implement the Mithril signer functionality in the Amaru node. High adoption of the Mithril signer is critical for ensuring protocol liveness and security making its implementation in new node versions essential. Amaru, being implemented in Rust, will allow the Mithril signer to be shipped as a default opt-out feature. This work will build upon the preliminary development of the Mithril signer, Mithril relay, and DMQ Rust libraries.