Parse the specified Node and decorate the supplied
BeanDefinitionHolder, returning the decorated definition.
The Node may be either an Attr or an
Element, depending on whether a custom attribute or element
is being parsed.
Implementations may choose to return a completely new definition,
which will replace the original definition in the resulting
The supplied ParserContext can be used to register any
additional beans needed to support the main definition.
source - the source element or attribute that is to be parsed
definition - the current bean definition
parserContext - the object encapsulating the current state of the parsing process
the decorated definition (to be registered in the BeanFactory),
or simply the original bean definition if no decoration is required.
A null value is strictly speaking invalid, but will be leniently
treated like the case where the original bean definition gets returned.