Add option to enable groups for oidc connectors

There's been some discussion in #1065 regarding what to do about
refreshing groups. As it stands today dex doesn't update any of the
claims on refresh (groups would just be another one). The main concern
with enabling it is that group claims may change more frequently. While
we continue to wait on the upstream refresh flows, this adds an option
to enable the group claim. This is disabled by default (so no behavioral
change) but enables those that are willing to have the delay in group
claim change to use oidc IDPs.

Workaround to #1065