feat(pwa): enable SW file caching

[adekbadek]

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• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Replaces the Cache-Control header used for PWA'a service worker file, so that it can be cached.

Also moves the PWA plugin integration file to a more applicable place.

How to test the changes in this Pull Request:

1. On trunk
2. $ curl -I <site-url>/wp.serviceworker, observe the cache-control: no-cache header
3. Switch to this branch, repeat, observe the Cache-Control: max-age=86400, must-revalidate header sent

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[github-actions]

Hey @, good job getting this PR merged! 🎉

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