Allow syncing a group against all providers at once

## What does this MR do?

Allows `Sync::Group` to sync members for a single group across all providers at once. Closes #399 

## Are there points in the code the reviewer needs to double check?

No

## Why was this MR needed?

The current implementation of LDAP group sync manages members based on a single LDAP provider. This is optimal because we can open a single connection to a given LDAP server (provider) and run all queries needed to sync all GitLab groups with links to that provider. If we want to sync a single group at a time we need to flip that model - for a single group, run the sync for all providers. 

This presents a few challenges since we have a state machine on the group. In the former/current model we need to set the state inside the loop. However, if we do that in the latter model we will likely run into a race condition. At least, it will execute unnecessary queries against the database. Other than adding the functionality needed to sync a single group, the MR moves the state machine handling to the respective class method. 

## What are the relevant issue numbers?

#399

See merge request !636