Add additional element callbacks to repository API

It would be very useful to have the ability to specify additional callbacks and actions to be added to the repository API, so that it is displayed alongside the forms from any repository plugin.

The form template objects passed into the filepicker could callback to plugins to inject additional form elements, and then contain additional validation and submission actions to handle the additional input in the forms.

The main use case here is to add a sort of "custom fields" style implementation, where additional file data can be specified, such as Department, Categorisation etc that goes beyond the upload context, and allows for this data to be stored at upload time in whatever manner a plugin decides.