Provide a public new-queue count endpoint

[davidperezgar]

Summary

• Adds a public GET /plugins/wp-json/plugins/v1/stats endpoint.
• Returns only the aggregate number of plugins awaiting initial review.
• Removes the need for a user-scoped Application Password to retrieve this metric.
• Adds REST API coverage for the endpoint.

Testing

• php -l on changed PHP files
• PHPCS on new files

Closes #758

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props davidperez, obenland.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[obenland]

@davidperezgar Looks like this needs a rebase.

There's no other endpoint this can be folded into?

[davidperezgar]

Rebased onto the current trunk. I also folded this into the existing Internal_Stats API: the public endpoint is now GET /plugins/wp-json/plugins/v1/stats and returns only queue_count. The existing POST /update-stats endpoint remains authenticated for writes.