feat: add tag parsing for feed source generated by endpoint

[Soare-Robert-Daniel]

Closes #899

Summary

When full_content magic tag is present, the endpoint will create a single feed with all the data. The feed is enhanced with custom tags under feedzy tag namespace.

With the changes from https://github.com/Codeinwp/feedzy-full-post/pull/89. The item feed looks like this:

<entry>
	<guid>https://community.511tactical.com/5-11-announces-the-return-of-abr-academy-classes-for-2023-in-collaboration-with-uscca-copy/</guid>
	<link href="https://www.511tactical.com/community/5-11-announces-the-return-of-abr-academy-classes-for-2023-in-collaboration-with-uscca-2/"/>
	<title><![CDATA[5.11 Tactical to Celebrate 20th Anniversary]]></title>
	<summary type="html"><![CDATA[
Celebrating its 20th anniversary, 5.11 Tactical reflects on its journey from rock-climbing pants to a trusted provider of Purpose-Built Gear™ for public safety professionals and outdoor enthusiasts. CEO Francisco J. Morales expresses gratitude to customers and reaffirms the company's commitment to empowering individuals to "Always Be Ready" through dependable, innovative gear.
The post 5.11 Tactical to Celebrate 20th Anniversary appeared first on 5.11 Community.]]></summary>
	<content type="html"><![CDATA[]]></content>
	<dc:creator><![CDATA[]]></dc:creator>
	<published>Sat, 28 Jan 2023 07:43:26 +0000</published>
	<link rel="enclosure" href="https://community.511tactical.com/wp-content/uploads/2023/01/2301_MK_20thAnniv_SocialAssets_3x2.webp" type=""/>
	<full-content type="html"><![CDATA[]]></full-content>
	<feedzy:parent-source>5.11 Community</feedzy:parent-source>
	<error></error>
</entry>

Changes:

• Added parsing for feedzy:parent-source tag into the feed.

Testing

• Install the Pro plugin
• Create a feed with multiple sources
• On the Map Content, in the Content field, add item_full_content and item_source tags.
• Import the feeds and check if the item source matches the article content.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step f#904 r you)

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