REST API: Add persistent caching to Comments Count endpoint

[chrisbliss18]

As discussed in #40587, the Comments Count endpoint does not offer use any caching layers. For some sights with large amounts of comments, this can lead to repeated high-cost queries when making multiple requests to this endpoint in a short amount of time.

This PR adds persistent caching support to this endpoint. This caching is limited to just sites that add a persistent cache plugins. Since the biggest sites and sites on managed platforms are likely to use such a plugin, we'll see some immediate benefit from this change. Further discussion will determine the best path forward to ensure that all sites (with or without a persistent cache plugin) can benefit from caching.

Fixes #40587

Proposed changes:

• Add use of persistent caching using WP_Object_Cache to data handlers for the Comments Count REST API endpoint.

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

Discussion in originating issue.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the performance/rest-api-comment-counts branch.

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[github-actions]

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[github-actions]

Code Coverage Summary

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