Using Postman to Authenticate to GraphQL

With the release of Sitecore 10.3, a whole bunch of new capabilities have come into GraphQL, such as the ability to create items and templates, do index rebuilds, publish items, and so forth.  But the documentation for setting up authentication to these services is somewhat intimidating, requiring you to build an MVC controller just to sign in.  In fact, it's pretty simple to set authentication with Postman too, which will be a familiar option for those who have worked with Sitecore Commerce.