Support for distributed groups claims on Azure

Support for distributed groups claims on Azure (#120)

* Added logic to handle if user is member of more than 200 groups, Azure specific.

* Changed return error to not be Azure specific

Moved azure-specific code to provider_azure.go, called using the
GroupsFetcher interface. Using oauth2.clientcredentials for
getting the Azure token. Error checking when talking to the ms
graph api. More constants, using the endpoint host from
_claim_sources for token scope.

Co-authored-by: Lasse Gaardsholt <[email protected]>