refactor: introduce strum::EnumIter to protobuf's NodeRewardType

[pierugo-dfinity]

This PR adds strum::EnumIter to NodeRewardType to make it future-proof to iterate over all its variants in case a new one gets added (mostly done in tests for now).

[github-actions]

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

2. Are there BREAKING changes?

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GOVERNANCE_CHECKLIST_REMINDER_DEDUP

1. No need, chore change in test code.
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