Add support for extension OAuth 2.0 Authorization Grants

[nohorbee]

Add a property to the OAuth 2.0 settings extensionAuthorizationGrants, which should be a list of any extension Authorization grant types supported by the Authentication server, since these can be anything that the OAuth vendor has defined, it is the vendor's responsibility to document how those should work.

#%RAML 0.8
title: My Sample API
securitySchemes:
    - oauth_2_0:
        description: |
            OAuth 2.0 implementation.
        type: OAuth 2.0
        settings:
          authorizationUri: https://www.myapi.com/1/oauth2/authorize
          accessTokenUri: https://www.myapi.com/1/oauth2/token
          authorizationGrants: [ authorization_code, password ]
          extensionAuthorizationGrants: [ 'urn:ietf:params:oauth:grant-type:saml2-bearer' ]

[aldonline]

Specific extensions like these are a good fit for annotations ( #174 )

[usarid]

Let's not add another key (not add extensionAuthorizationGrants), and instead allow the items in the value of authorizationGrants to include not just the standard authorization_code, password, client_credentials, and implicit strings but also any absolute URI, just as defined in https://tools.ietf.org/html/rfc6749#section-4.5 . There will never be any ambiguity in the values of the items, because no absolute URI looks like any of the strings authorization_code, password, client_credentials, and implicit.

An example:

authorizationGrants: [ authorization_code, password , 'urn:ietf:params:oauth:grant-type:saml2-bearer' ]