Support userinfo endpoint as fallback for groups

[santoru]

The current implementation to retrieve groups of a user to set its permission role (staff, admin) is broken and there are multiple issues open ( #8 #14).

From my knowledge, Okta does not return the groups claim by default unless it is explicitly configured in the web UI. This solution however does not seems to work as Okta only returns groups ownership via the userinfo endpoint and not via the token endpoint.

The proposed solution applies a fallback request to the userinfo endpoint if the groups claim is not found using the token endpoint. This solution does not change the current implementation but only try to retrieve the groups claim from a new endpoint if it's not present on the token endpoint.

For the sake of reusability, a new generic function to interact with the userinfo endpoint was added, even if at this point in time only the groups claim is used.