Allow extra args to be provided to the OIDC auth endpoint

[alanwilkie-finocomp]

Some OIDC Identity Providers provide extended capabilities by adding extra query string arguments to the authentication request.

Specifically, Keycloak allows a default identity provider to be specified by adding a "kc_idp_hint" parameter to the authentication request (see https://www.keycloak.org/docs/latest/server_admin/#_client_suggested_idp).

It would be great to be able to include extra request arguments provided by the configuration.

Proposed changes in PR #67.