Kernel/SDK: impl Signature Verifier for PublicKey

Context

Now that the kernel SDK exposes the enum PublicKey, it would be nice for this type to implement signature verification. This MR implements SignatureVerifier for the PublicKey type.

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Manually testing the MR

cd src/kernel_sdk
make test

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR