OAuth authentication with third party provider

This epic will cover a series of improvements required to oAuth module for being able to use the Nuxeo Platform token store for some oAuth authentication use cases:

• ability to define that a token is shared to other users (a visibility scope would be useful I believe)
• ability for a given user to generate a token for a given service. Probably Administrator should be able to declare services and their authentication endpoint so that it is then proposed to the user . Or it could be declared by contribution (since the url can be considered as part of the code of the connector, not a problem). The user could choose in a drop down a service and generate a token for it.

• apis to use those tokens easily (if not there already)