Set user's tenant

This route is for the vendor to set the active tenant of a user. The active tenant is the tenant the user will see in their admin portal and also the tenant for which the API reference will default to in situations where a route is tenant specific. Send the user ID as a path param and the tenant ID as a PUT body param. When using a non-existing tenant ID, there will be a tenant create for the provided ID. A vendor token is required for this route, it can be obtained from the vendor authentication route.