Allow clearing default Azure and Google auth code options (#4315)

Allow users to clear the default IdP auth code options, by explicitly
setting an empty idp_request_params map.

To do this in a YAML config file, set:

    idp_request_params: {}

internal/identity/oidc/azure/microsoft.go

@@ -61,7 +61,7 @@ func New(ctx context.Context, o *oauth.Options) (*Provider, error) {
 	p.Provider = genericOidc
 
 	p.AuthCodeOptions = defaultAuthCodeOptions
-	if len(o.AuthCodeOptions) != 0 {
+	if o.AuthCodeOptions != nil {
 		p.AuthCodeOptions = o.AuthCodeOptions
 	}