Allow select2 widget to pass additional dynamic parameters when calling automation operation

Currently document suggestion widget based on select2 only pass what is typed in the input by the user as automation parameter which will be used as a page provider parameter.

However, many use case requires to pass additional page provider parameters such as previously filled inputs in the form.

We should add a property to the widget which will take the name of a javascript function which will return additional parameter values to be passed to the operation.

This function will be called each time a call is done to the server.

A typical use case is a suggestion widget of which the suggested entries depend on a first field. The js function can easily retrieve the value in the DOM client side and return it to select2 which will pass it on to the server.