feat: [DiscoveryEngine] add Natural Language Query Understanding fields and messages to DiscoveryEngine request, response, and datastore messages (#8915)

* feat: add Natural Language Query Understanding fields and messages to DiscoveryEngine request, response, and datastore messages
feat: add CrowdingSpec to SearchRequest to set crowding settings
feat: add output-only field SemanticState to SearchResponse.
docs: update documentation for search and data store NLQ features
docs: other misc documentation updates

Clients can specify Natural Language Query Understanding-related fields, as well as CrowdingSpec. These features are available in the V1 APIs.

PiperOrigin-RevId: 868345232

Source-Link: googleapis/googleapis@56bc1b4

Source-Link: googleapis/googleapis-gen@cee5877
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