OpenId Provider endpoint metadata should be allowed as link rel values

[barmintor]

While the manifest spec may wish to be agnostic on auth mechanisms, it would be useful to permit a manifest or OPDS feed to indicate some service endpoints relevant to authorization as link metadata. The OpenID specs enumerate a few such values in their provider discovery metadata:

• token_endpoint
• authorization_endpoint
• userinfo_endpoint

[HadrienGardeur]

This would be handled in properties under the current Link Object model.

IMO this is the wrong repo to handle this (it would fall under the work covered by OPDS Authentication) but this could be referenced from this repo: https://github.com/readium/webpub-manifest/blob/master/properties.md