How to extend REST API?

Question

API

One service has its own REST API. I would like to use it for Microsoft Flow in PowerApps (Logic Apps). There, to connect the REST API, you need to load the swagger description of the REST service, and here the main problem arises. Swagger supports three types of authentication: http basic, oauth, apikey - and the original service API uses its own type of authentication. And it is necessary for the service to write something like a proxy that implements authentication supported by swagger.
And the question is, are there any ready-made solutions for this?