Vertex AI Search for commerce: Conversational product filtering

## Feature **Vertex AI Search for commerce: Conversational product filtering** As part of Search for commerce's Guided search package, `ConversationalSearchSpec` sits on top of the Vertex AI Search for commerce. When coverage parameters are met, Search for commerce users can enable this feature in the console or by setting the `followup_conversation_requested` flag to `true` in the search service interface. Conversational product filtering uses an LLM-generated question for each catalog attribute where `allowed_in_conversation` field is enabled. For more information, see [Conversational product filtering](https://cloud.google.com/retail/docs/conversational-filtering).