[SCIM2] user extensions not keyed by URN

[hauntingEcho]

Currently, the EnterpriseUser key is used in SCIM user creation to specify the properties of a user specified in urn:ietf:params:scim:schemas:extension:enterprise:2.0:User.

Per RFC 7643 section 3.3, "Except for the base object schema, the schema extension URI SHALL be used as a JSON container to distinguish attributes belonging to the extension namespace from base schema attributes.". See also the example User JSON in section 8.3.

This causes compatibility issues with SCIM client libraries.