Add pluggable eval metric registry architecture

[AroniasPrenovost]

Foundational RI-7 roadmap piece: a registry-based interface so new eval metric types (LLM-as-judge, citation accuracy, schema validation, etc.) can be added by implementing EvalMetric and registering it, without touching existing metrics or the eval pipeline core.

Description

Type of change (check as many as applicable)

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation
• Maintenance (version updates, config changes, etc)
• Third-Party/Dependency integration change (schema changes, new/removed/updated APIs)

Work Remaining

How Has This Been Tested?

Pull Request Checklist

• I have added this PR to the CHANGELOG.md where most applicable
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