Derive some traits for ed25519::{PublicKey, Signature}

[thibault-martinez]

Description of change

Derive some common traits for public types. Without the traits, these types are basically unusable by bee.

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[arnauorriols]

What's the status of this PR? IOTA-Streams would be interested in it getting merged 🙂

[thibault-martinez]

Last time I checked, some trait impls were missing in the dependencies and the owners didn't reply, I'll try to check again

[arnauorriols]

We could separate the serde traits in a different PR, and get the basic ones merged.