Provide a public new-queue count endpoint

[davidperezgar]

Summary

• Adds a public GET /plugins/wp-json/plugins/v1/stats/new-queue-count 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]

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Props davidperez, obenland.

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[obenland]

@davidperezgar Looks like this needs a rebase.

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