Sync groups and group membership through auth strategies

First off: It seems there is already a few feature requests on https://wiki.js.org/feedback/?search=group for this, so I added a ticket because I'm thinking about helping with the first implementation here.

Actual behavior

Currently authenticating with different strategies does not update group membership, even if the strategy supports that (i.e. roles in Keycloak, groups in LDAP). Manually adding users to groups is cumbersome and makes it difficult to use Wiki.js in larger teams where you want give some sub-teams their own private section.

Expected behavior

Signing in with a strategy that supports group / role memberships should create a group on Wiki.js, if it doesn't exist yet, and then add the user to the group during authentication. There should be settings in the admin UI of the different strategies that support groups, to control this behavior. My guess would be:

• Toggle "Synchronize groups"
• Toggle "Synchronize group membership"
• Group search query for LDAP

You could get infinitely more complex with custom group mappings, background sync of groups from LDAP, nested groups, permission mapping, etc, but as a first version the above seems useful.

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I'd be happy to dig into the code and try to contribute a PR for LDAP and/or Keycloak, if you agree that this is a useful feature @NGPixel - it seems widely requested on Canny.