Add Native App Rustice query service function

[osipovartem]

Summary

• add APP_PUBLIC.RUSTICE_QUERY(sql_text STRING) to the Native App startup flow so customers can run simple Rustice smoke queries from Snowsight Worksheets or regular snow sql
• add a Snowflake service-function endpoint at /snowflake-function/query that accepts Snowflake service function batch payloads and returns the Rustice Snowflake REST-style response as JSON text
• pass the configured client database/schema into the container so the service function runs with the same default context as the consumer quickstart
• update Native App quickstart, packaged app docs, customer worksheet SQL, and security evidence to document the new route

Verification

• cargo +stable test -p api-snowflake-rest test_native_app_service_function_query -- --nocapture
• snow --config-file /home/artem/.snowflake/config.toml app bundle from deploy/native-app

Notes

• RUSTICE_QUERY(...) is intended for smoke tests and simple UI checks. Normal Rustice SQL/dbt workloads should still use embucket-snow through the SPCS ingress endpoint.
• The currently installed/published app version must be rebuilt and published as a new patch before consumers see this function.
• The clean consumer run is still blocked on trial-account SPCS support: 509009 External access is not supported for trial accounts.