Always check XML Set elements with property attribute

[gregw]

Target Jetty version(s)

= 10.0.x

Enhancement Description

Since jetty-10, the XML Set element could have a property attribute that only does the set if the property is set. The problem with this is that the code may change and we do not detect that the XML is now wrong unless somebody attempts to set that property. The XmlConfiguration class should be change to always check that at least one setter exists for a Set element with a property attribute.

[janbartel]

@gregw also, the documentation needs updating for the Set element, because it doesn't mention the property attribute at all. See https://www.eclipse.org/jetty/documentation/jetty-10/operations-guide/index.html#og-xml-syntax-set