Bootstrap Mithril Signer node

[jpraynaud]

A Mithril Signer node needs to be created

• Bootstrap a Mithril Signer node in mithril-network/mithril-signer
• Implement basic CLI interface
• Implement configuration handling (config file + env vars)
• Implement the slog crate for structured logging (see ADR)
• Implement Dockerfiles
• Implement README
• Implement Makefile
• Add new jobs in CI (build/test, docker, doc)