feat(nns): Implement self-describing for Register/Deregister KnownNeuron

[jasonz-dfinity]

Implement the conversion to SelfDescribingAction for RegisterKnownNeuron and DeregisterKnownNeuron

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

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Reference(s)

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