[Task] Upgrade to Ed25519 Signature 2020

[eike-hass]

Description

Add support for Ed25519VerificationKey2020 ~and Ed25519Signature2020.~ We still need to keep Ed25519VerificationKey2018 for backwards compatibility, but should default to Ed25519VerificationKey2020 in the future.

Motivation

Stay compliant with the latest specification.

Resources

https://w3c-ccg.github.io/lds-ed25519-2020/

Change checklist

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

• [ ] The feature or fix is implemented in Rust and across all bindings whereas possible.
• [ ] The feature or fix has sufficient testing coverage
• [ ] All tests and examples build and run locally as expected
• [ ] Every piece of code has been document according to the documentation guidelines.
• [ ] If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
• [ ] If the feature is not currently documented, a documentation task Issue has been opened to address this.

[cycraig]

We use JcsEd25519Signature2020, we do not use Ed25519Signature2018 nor Ed25519Signature2020 because they have a different data canonicalisation algorithm (RDF).

[eike-hass]

@cycraig Thank you for clarifying 🙏As there is no superseding spec should we consider this issue obsolete?

[cycraig]

We should still add first-class support for Ed25519VerificationKey2020 (and maybe default to it) for interoperability. It's only the signature I think we should ignore, considering the better alternatives.

[eike-hass]

General cryptographic agility is now tracked in https://github.com/iotaledger/identity.rs/issues/1037.