Evaluation: Track evaluator usage through user agent

[ninghu]

Description

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• [ ] The pull request does not introduce [breaking changes]
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Testing Guidelines

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[ninghu]

For the time being, Multi-modal is not covered in this PR, as they may be integrated into content safety evaluators later on. Feature 3594503: Merge multi modal evaluators into regular evaluators

[azure-sdk]

API change check

API changes are not detected in this pull request.