[FEATURE] Authman admin group for stem

[shurwit]

Is your feature request related to a problem? Please describe.
Currently, there is no way for the admins of an Authman stem to set the default admins for all groups with that stem.

Describe the solution you'd like
A clear and concise description of what you want to happen.

We should allow for the creation of an Authman group with a given stem that contains the default admins to be set for all other groups with the same stem. This group name should be configured within the ManagedGroupConfigs. This group must be ignored when auto-creating Group BB groups based on any groups with the specified stem.

Describe alternatives you've considered
An alternative would be to create specific permissions that allow someone to set the list of admins for a given Authman stem only. This would be complicated, confusing, and less convenient for the admins.

Additional context
All other default admin mechanisms should remain unchanged.