[FEATURE] Improve managed group admin assignment and synchronization

[shurwit]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Currently we use a combination of admin UINs defined in environment variables and in Authman group descriptions to determine which members should be admins. We would like to make this system more flexible to allow these admins to be managed independently for each set of groups and updated at any point.

We would also like to have the Groups BB manage periodically synchronizing the groups with Authman internally rather than relying on an external task to call an API.

We would also like to ensure that when these admins are updated, the changes are reflected the next time the group is synchronized.

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

• We should create a new collection of Authman group stems that will be synchronized which include any admins for these groups. We should expose admin APIs to add/update/delete documents in this collection.
• When group synchronization occurs, we should check for any changes to the documents in this collection and ensure that the group admins are updated appropriately to match any added or removed admins.
• We should also create a timer that calls the SynchronizeAuthman function periodically so we can remove the external task calling the API to trigger synchronization.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

One alternative discussed was to create an admin API which allows the admins of each group to be set. This is somewhat less convenient as it would require this data to be manually added to each group rather than automatically set when the group is created and first synchronized. Note that we may still want a similar API in the future to allow the default admins created by this process to add other admins to specific groups.

[shurwit]

I have opened #213 to address the removal of admins, so that is no longer in scope for this issue

[shurwit]

Related additional feature described in #214