Allow creating service users at platform level

[kushsharma]

Is your feature request related to a problem? Please describe.
Currently, service users can only be created inside an org but we should have the option to create service users in platform entity as well.

Describe alternatives you've considered
You can create an org yourself as an internal org and create service users in it. After that add that service user as a platform member and it will be same as above but with one extra step of creating a dummy org.