Basic authentication and endpoint calls.

• Export a function to generate R/.R and tests/testthat/.R files to authenticate the user and make a call to the API, given the URL of an OpenAPI spec in YAML format. The generated files will follow and encourage best practices, and will serve as the core around which the rest of a package would be built.
• Produce a vignette about configuring authentication.
• Potential challenges: Authentication is a complex and delicate subject. Some APIs require registration of special apps to “catch” authentication requests, while others simply provide an API key. I will need to carefully navigate these complexities in the vignette.