Mithril Signer register keys w/ Aggregator

[jpraynaud]

The Mithril Signer must be able to register its verification keys with the MIthril Aggregator:

• Implement http handler for /register-signer route in Mithil Aggregator
• Implement http handler for /certificate-pending route in Mithil Aggregator
• Remove fake signer registration from Mithril Signer
• Implement real signer registration in Mithril Signer
• Implement typed errors where missing
• Remove usage of secret key