Implement custom grant type client_authenticated_user

[stevehu]

For this grant type, the client application(normally will be web server) is responsible for authenticating user (single sign on most of the case) and send user info to the token endpoint to get access token. The passed in user info must contain the following fields.

userId
userType

Any other fields will be added to the JWT claim automatically.

This is the requirement from one of our clients as they have single sign on already on their web server and need to get a token to access APIs.