Add OAuth2 as a potential authorization layer

[awoie]

I suggest to support other authorization schemas as well. OAuth2 is still the most prevalent mechanism for web services. OAuth2 support should be added. A similar approach to SDS could be taken.

From the SDS spec:

The system is expected to specify one mandatory authorization scheme, but also allow other alternate authorization schemes. Examples of authorization schemes include OAuth2, Web Access Control, and [ZCAP]s (Authorization Capabilities).

@msporny @dlongley @OR13 can you please clarify whether you intend to support ZCAPs only.

[OR13]

Its easy to add support for OAuth2, its handled the same way ZCaps are handled... in the http request pre handler (or some similar function depending on the framework).

I think from a spec perspective, we could easily support both, assuming someone contributed the OAuth2 spec material.