Update user API should work with loginId or userName+provider

[kochhar]

Presently the updateUser API requires a userId to update a user. The userId is generated when the user record is created. This means that if partners need to update a user, they will need to store this userId in their system.

Ideally, we should not be exposing our unique user-identifier to external systems. This can create a dependency on our data limiting our flexibility.

Suggest that the updateUser API should work primarily with the user's loginId. Alternatively, we can also accept the userName+provider and generate the loginId within our system.