define the access token provider interface for ruby

[baywet]

In the past few months we've splat the authentication provider for bearer auth into multiple components:

• the base bearer authentication provider, which is in charge of formatting the authorization header
• the allowed hosts validator, which verifies the request targets a known host before performing authentication
• the access token provider, which gets a JWT access token. To be used in combination with the base bearer token provider

example

Those changes have not been reflected to the ruby abstractions are a prerequisite to the authentication provider implementation work listed in #421.

More information with #1009