Boost: Save concatenated JS/CSS in files on server

[dilirity]

Related #41035

Proposed changes:

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?
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Jetpack product discussion

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

Testing instructions:

• Go to '..'

[github-actions]

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