Protect: Add Scan tab threat count

[dkmyta]

Descriptions

Conditionally adds a count of active threats beside the Scan tab, when threats exist.

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?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

• https://github.com/Automattic/jetpack-scan-team/issues/1514

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

• No

Testing instructions:

• Use the Jetpack Beta Tester using this branch on Protect on a site with existing threats and a Scan upgrade
• Verify that the count is found next to the Scan tab
• Remove all threats and ensure a count is no longer present
• Downgrade and ensure the count logic remains

Screenshots

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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Protect plugin:

• Next scheduled release: none scheduled.

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