feat: [Firestore] Expose the Database.firestore_data_access_mode parameter (#8900)

* feat: Expose the `Database.firestore_data_access_mode` parameter
feat: Expose the `Database.mongodb_compatible_data_access_mode` parameter
feat: Expose the `Database.realtime_updates_mode` parameter
feat: Expose the `Index.unique` parameter
docs: Update documentation on several parameters, especially when relating to the new `enterprise` database edition

PiperOrigin-RevId: 865444806

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