Connectors that don't support refresh tokens shouldn't return one instead of erroring

[ericchiang]

If a connector doesn't implement connector.RefreshConnector it should silently not return a refresh token when offline_access is requested instead of erroring out, like our current SAML connector.

For backwards compatibility we'll have to make the OIDC connector implement this method, so it continues to support refresh tokens, despite not being able to refresh the users identity (#863).

// Refresh is implemented for backwards compatibility, even though it's a no-op.
func (o *oidcConnector) Refresh(ctx context.Context, s connector.Scopes, identity connector.Identity) (connector.Identity, error) {
    return identity, nil
}

cc @rithujohn191