Add function to associate an integration with a piece of content

The recipe [Associate Integrations with Content](https://docs.posit.co/connect/cookbook/content/updating-content-associations/index.html) uses a `client$GET()` call. Replace it with a function.

Something like `content_add_integration(content, integration)`, taking a `Content` and an object returned by code implemented in https://github.com/posit-dev/connectapi/issues/413.