Creates a new Basiq user object
A user must be created before connecting to any financial institutions, as this is where data will be aggregated.
We do not check for existing users with the same details
We rely on the ID of the user for uniqueness, so will not do a check for existing users with the same mobile or email. If this is a concern then it should be handled in the partner application. You must only provide a mobile OR an email to create the user - not both.