Swap ed25519-dalek for ed25519-zebra

[arjanz]

Background

Article at https://github.com/MystenLabs/ed25519-unsafe-libs mentions potentially unsafe ed25519 signature libraries that allow a public api where secret and public key can be provided independently as signing function inputs. Misuse of these public apis can result to private key exposure.

Solution

Following Substrate on using ed25519-zebra bindings in stead of ed25519-dalek: https://github.com/paritytech/substrate/pull/11781