Allow XML elements to pass through as field values to process plugins

Problem/Motivation

I have complex data structures in an XML export that I need to run through a custom process plugin which will upload files etc then save entities and return those entity ids as part of an entity reference field. In my migrate_plus migration config I have the field set to an attachments field which looks something like this:

<attachments>
			<attachment1>
				<title><![CDATA[Document 1]]></title>
				<url><![CDATA[example.com/document1.pdf]]></url>
			</attachment1>
			<attachment2>
				<title><![CDATA[Document 2]]></title>
				<url><![CDATA[example.com/document2.pdf]]></url>
			</attachment2>
</attachments>

But my process plugin receives an empty string because the Xml parser casts the field values as strings.

Proposed resolution

Check at a field level if we have complex data and cast appropriately.