Allow the UI list of Provisioners to be extended

The drop down list of provisioner types that can be created is a hard coded list of specific classes in ProvisioningConfiguration.provisionerConfigClassNames that is defined as final at Grouper startup. If an institution creates a custom provisioner, the classes can be supplied in a separate jar. But there isn't a way to extend the drop down list since it is sealed.

Maybe something like this in grouper-ui.properties?

additionalProvisioners.0.class = MySpecialProvisionerConfiguration

additionalProvisioners.1.class = OtherSpecialProvisionerConfiguration

The provisioners would then appear after the built-in ones, in the order enumerated in the config.