Derive serde::Serialize and serde::Deserialize for PublicKeyEd25519

[lassemoldrup]

Purpose

Needed for https://github.com/Concordium/concordium-rust-smart-contracts/issues/422.

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.