An action that will create a form instance, let it handle the current request (and
do validation), and will return not-null if validation was successfully or null when
validation failed. In both cases, the created form instance is stored in a request attribute,
so that it can be picked up later on by other components.
Required parameters:
form-definition: filename (URL) pointing to the form definition file
attribute-name: name of the request attribute in which the form instance should be stored