Add the capability to rename the superuser role currently called 'admin'

This could be useful for people switching from REST to Hasura. We're initially switching our reads over, and would like to use the same named role for admin/superuser that we do on our REST side.

I suppose you could rename the role to 'admin' in the Hasura section of the JWT but it would be nice to use the same name in both places to avoid dev confusion. Currently we use `admin` for another type of role and have a `superuser` permission which has access to everything.

Tanmai asked me to log this issue during a recent webinar.

The issue names no files or tests. Begin by tracing where the Hasura JWT role and built-in `admin` superuser role are defined; done means the superuser role can use a configured alternate name without losing its existing access.

Written by the indexing model from the issue text.