feat(core-api): add sorting to voters endpoint

[dated]

Proposed changes

Adds support for ordering to the delegates/{id}/voters endpoint and returns the voters ordered by balance:desc by default.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

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