Block Issuer Keys interface

[PhilippGackstatter]

Description of change

Add a Block Issuer Keys interface that is implemented by Ed25519 keys. This is so we can add a type prefix to the serialized public key and are more future proof.

Type of change

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• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

Existing tests run successfully. Manually checked the produced JSON has the expected type prefix.

Change checklist

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