feat(rss-feed): Add Feed enhancements for Republications.

[Takshil-Kunadia]

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:

Addresses https://app.asana.com/0/0/1209344511155915/f .

1. Add relevant options to the RSS Feed settings, if the Republication tracker plugin is activated:
   • A checkbox to add a republication tracker snippet to posts.
   • A checkbox to filter the feed to only include republishable posts.
   • A select field to choose between RSS and Atom feed formats.
2. When the user selects the Atom feed format, the "Wrap the content of <title> elements in CDATA tags" option is disabled and forced to false (since WordPress Atom feeds encoded title by default).
3. Adjusts the feed URL generation and query modifications to reflect the selected feed format.
4. Outputs the <republication_tracker> tag in the feed when the tracker option is enabled.
5. Modifies the meta query so that only posts not marked to hide the republishing button are included when filtering is active.

How to test the changes in this Pull Request:

1. Install & Configure Republication Tracker Tool.
2. Create Feeds from Admin > RSS Feeds.
3. Check for new options.
   • Checkbox for adding republication tracker pixel.
   • Checkbox for only including republishable posts.
   • Select for RSS or Atom feeds.
4. Try the above settings and check the feed link generated.
5. If republication tracker pixel checkbox is clicked a <republication_tracker> tag is included adding the tracking pixel.
6. If only Include Republishable posts checkbox is added, only posts which have "Hide Republishing Button" metabox checkbox unchecked will be included.
7. RSS & Atom feeds will be displayed based on the selected feed type.

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?

[Takshil-Kunadia]

Need to do this as WordPress atom feed wraps title in CDATA by default.