OAuth2 Authentication

This is a followon to linked ticket MGNLRESTCL-44, and can perhaps leverage the work already implemented there. 

User Story

As a developer, I want to connect to REST endpoints which require OAuth2, so that I am able to connect to the endpoints that I need.

Acceptance Criteria:

• Automatically request a new token whenever the active one expires.

Note:

Oauth2 is a common, modern, protocol required by many enterprise endpoints.

Such as:

• Amadeus https://developers.amadeus.com/self-service/apis-docs/guides/authorization
• Marketo https://developers.marketo.com/rest-api/authentication/
• Commercetools https://docs.commercetools.com/http-api-authorization
• Salesforce https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api/intro_understanding_oauth_endpoints.htm#!
•  ...

 Working example for Amadeus exists; review against others.