Lowercase group claims from the oidc provider

As humans create groups in e.g. azure AD the capitalisation can end up somewhat arbitrary. As for the intended downstream policies all groups are expected to be fully lowered-cases enforce this on the claims as well.

If in future some mediator users would like a different policy then configuration option could be added, but for now just always enforce this.

Signed-off-by: Sjoerd Simons sjoerd@collabora.com