CLI: New commands rill sudo org join and rill sudo org list-admins

[begelundmuller]

This PR adds two commands to make it easier to manage orgs as a superuser:

• rill sudo org list-admins <org>: lists all the admins of an organization. This can be used together with rill sudo user assume to temporarily assume an admin role on an organization that allows access from superusers.
• rill sudo org join <org>: adds the current user as an admin of an organization. This action is not temporary and the user will show up in member listings.

Closes https://github.com/rilldata/rill-private-issues/issues/624

[Kavinjsir]

LGTM!
Just left a small question to understand the coding pattern we are using :)

[pjain1]

Should we just error in case of role change of already added user and indicate to use set-role cmd. Earlier behaviour was incorrect in case of role change that it would do nothing and still send out an email with wrong role name. I am fine with this approach as well.

[begelundmuller]

Should we just error in case of role change of already added user and indicate to use set-role cmd. Earlier behaviour was incorrect in case of role change that it would do nothing and still send out an email with wrong role name. I am fine with this approach as well.

It was a product wish to not error in rill user add even if they were already added, so given that constraint I think this behavior is closer to what's expected.