Boost: Add cloud critical CSS generation feature

[haqadn]

Fixes 45-gh-Automattic/boost-hydra

Changes proposed in this Pull Request:

Jetpack product discussion

pc9hqz-fv-p2

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

Testing instructions:

• Go to '..'

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Jetpack plugin:

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Boost plugin:

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• Scheduled code freeze: February 21, 2022.

[haqadn]

Closing in and continuing fresh on #22885