Add attributes to an OrganizationMembership

[xgp]

model changes:

• create an OrganizationMembershipAttributeEntity (name value pairs) and add it to the OrganizationMembershipEntity as a collection
• create an OrganizationMembershipModel that includes the new attributes and the roles
• add a OrganizationMembershipModel getMembershipDetails(UserModel user) method to OrganizationModel
• duplicate the getMembersStream and searchForMembersStream methods in OrganizationModel with methods that return streams of OrganizationMembershipModels.

resource/representation changes

• in the MembersResource, update the GET method to return a UserRepresentation with a membership object containing attributes collection and roles collection.