Allow extension to use full path expression to a node in XML

[w3stling]

For Atom feed name element can exist inside both author element and contributor element. Allow to use full path expression/feed/entry/author/name and /feed/entry/contributor/name in extension to distinguish the two apart.

Atom feed example:

<feed>
...
    <entry>
        <title>An entry from my feed</title>
        <summary>DEFAULT --- This is the default summary</summary>
        <link rel="self" href="/web20/sample_atom_feed/entry" /> 
        <author>
              <name>Joe Bloggs</name>
              <uri>http://www.hursley.ibm.com/JBloggs/</uri>
              <email>[email protected]</email>
        </author>
        <contributor>
              <name>John Doe</name>
        </contributor>
        <category term="Comments" />
        <published>2023-08-10T15:41:00</published>
     </entry>
...
</feed>

Custom extension that maps value Joe Bloggs to field Comments in Item object using full path expression:

List<Item> items = new RssReader().addItemExtension("/feed/entry/author/name", Item::setComments)
                                  .read(URL)
                                  .collect(Collectors.toList());