Returns an instance of SparkApplication which on which init() will be called.
Default implementation looks up the class name in the filterConfig using the key APPLICATION_CLASS_PARAM.
Subclasses can override this method to use different techniques to obtain an instance (i.e. dependency injection).
filterConfig - the filter configuration for retrieving parameters passed to this filter.
the spark application containing the configuration.
javax.servlet.ServletException - if anything went wrong.