feat(block-theme): move co-authors RSS feed code to the Newspack Plugin

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR and Automattic/newspack-theme#2671 remove functionality that adds Co-Authors to the RSS feed from the classic theme, and adds it to the Newspack Plugin. This makes it available regardless of theme used.

Closes NPPD-1345

How to test the changes in this Pull Request:

1. Apply this PR and feat(block-theme): move coauthors RSS function to the Newspack Plugin newspack-theme#2671
2. Set up some posts with a few different options: multiple authors, or guest authors.
3. View the site's RSS feed (/feed in Chrome to view; the same downloads it in Firefox, which also works!) and search for <dc:creator> in the posts you created.
4. Confirm that your bylines are respected (guest authors, multiple authors).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

This PR adds a Newspack-side integration for the Co-Authors Plus plugin so RSS/feed author output can include all co-authors, and wires that integration into the main plugin include list.

Changes:

• Added a Newspack\RSS_Feed integration that filters the_author in feed contexts to output co-authors.
• Included the new integration file in Newspack::includes() so it loads with other Co-Authors Plus integrations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
includes/plugins/co-authors-plus/class-rss-feed.php	Introduces a feed-specific the_author filter to return co-author output in feeds.
includes/class-newspack.php	Loads the new Co-Authors Plus RSS integration during plugin includes.

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