feat: [DiscoveryEngine] Add RelevanceFilterSpec to SearchRequest in v1 (#9333)

* feat: Add RelevanceFilterSpec to SearchRequest in v1

PiperOrigin-RevId: 945024989

Source-Link: googleapis/googleapis@71573e6

Source-Link: googleapis/googleapis-gen@ab055d4
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Author: gcf-owl-bot[bot]