How can I enable Async queries in a scratch org in Salesforce?

Hi,

Async SOQL is a method for running SOQL queries in the background over Salesforce entity data, including sObjects, BigObjects, and external objects (accessed via Lightning Connect). It provides a convenient way to query large amounts of data stored in Salesforce.
Async SOQL is implemented in the form of a RESTful API that enables you to run queries in the familiar syntax of the SOQL language. You can run multiple queries in parallel, and monitor their completion status.

For More you can take help from :-https://releasenotes.docs.salesforce.com/en-us/winter16/release-notes/rn_general_async_query.htm