Add support for the new API Tokens (beta) auth scheme

[hunts]

Description

API Tokens provide a new way to authenticate with the Cloudflare API. They allow for scoped and permissioned access to resources. This pull request added support for using it. An example and necessary introduction will be added to README by a follow up pull request once the open beta is announced.

Has your change been tested?

Tests a request send from the api client instantiated via NewWithAPIToken() contains the expected "Authorization" header.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[patryk]

cc: @jacobbednarz

[jacobbednarz]

Thanks! We've been running a patch similar to this to use with Terraform provider for the closed beta so big 💯 to landing this and getting it into the provider 🙇

[patryk]

\o/