CITATION.cff support

[effigies]

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To support CITATION.cff we need:

• [ ] Load CITATION.cff as YAML, fail if invalid
• [ ] Validate with JSON schema (raw link). We may need/want to maintain a fork of this to support roles until they include them.
• [ ] Populate context (schema validator) with relevant fields for validation checks (needs coordinated PR in spec).

xref https://github.com/bids-standard/bids-specification/issues/901 https://github.com/bids-standard/bids-examples/pull/426