feat: [DiscoveryEngine] Added ranking_expression_backed and rank_signals fields related to the Custom Ranking feature (#8579)

* feat: Added `ranking_expression_backed` and `rank_signals` fields related to the Custom Ranking feature
docs: A comment for field `ranking_expression` in messages `.google.cloud.discoveryengine.v1alpha.SearchRequest` and `.google.cloud.discoveryengine.v1beta.SearchRequest` is changed to support the Custom Ranking use case

PiperOrigin-RevId: 805306845

Source-Link: googleapis/googleapis@0ef98bc

Source-Link: googleapis/googleapis-gen@79a8688
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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]