VideoPress: Add endpoint for returning today's video stats

[lhkowalski]

Relates to #26479.

Changes proposed in this Pull Request:

• Register videopress/v1/stats endpoint to return general video stats
• Return the counter of plays for today, using the information provided by the WPCOM REST API
• The plan was to add a counter for all-time plays, but we still need to sort out how to request it, so it's not included here

Pending issues:

• which capability the user needs to have to see this data?
• how to make this work inside WPCOM / make sure the class got loaded (similar to #26406)

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?

Jetpack product discussion

N/A.

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

No.

Testing instructions:

• Use a site with VideoPress installed
• Do a GET request to yoursite.com/wp-json/videopress/v1/stats, for example:

curl -X GET -u "user:pass" http://yoursite.com/wp-json/videopress/v1/stats | jq

• The expected response is similar to this:

{
    "plays": {
        "today": 1
    }
}

• It's useful to have some media available on the site with usage information, so make sure you uploaded at least one video, put it on a post and do at least one play on it

[github-actions[bot]]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run bin/jetpack-downloader test jetpack add/videopress-stats-endpoint to get started. More details: p9dueE-5Nn-p2

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[lhkowalski]

we'd probably like to extract a method to get the stats to get it in the backend size without the need to hit the endpoint from the client

@retrofox I refactored the code to move the stats requesting to a new Stats class. Let me know what you think :+1: