Ask HN: API authentication as-a-service?

I'm building a public-facing API. I currently do authentication through AWS Cognito, though it's a bit painful (as probably not what Cognito is for) and also I don't like the idea of using uname/pwd.

I have seen a few API's where I thought workflow was quite nice. In example Esri ArcGIS Online API workflow is something I would like to have in my app.

Are there any services available (like Cognito) where this is available on as-a-service basis?