App verification

Adds message signing and verification to the REST API. (Closes Beezig/obs-controller#4)

Working:

• Basic Ed25519 implementation for message signing/verification
• App metadata + public keys serialization into a binary file
• HTTP middleware
• User interface to accept/reject apps

To Test:

• App registration via HTTP
  • Server generates private key for the app and sends it encrypted with a shared ECDH secret.

PoC client available at https://gitlab.com/Beezig/honeycomb/-/tree/master/obs-controller-client