Boost: Add cornerstone pages support

[dilirity]

TODO:

• Boost: Tag cornerstone pages with "experiment" #39945
• Boost: Indicate difference between free and premium for cornerstone pages #39948
• Boost: Rename to cornerstone pages #39949
• https://github.com/Automattic/boost-cloud/issues/632
• Boost: Fix some cornerstone page urls not having a leading slash after save #39967

Proposed changes:

• Add cornerstone pages feature.

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

n/a

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

no

Testing instructions:

• Setup Boost;
• Test the cornerstone pages UI to make sure it works as expected.

[github-actions]

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[haqadn]

Couple of requests:

• Feature flag is not removed. It's still hidden in the UI if the const is not defined.
• Can the tag be called "Experimental" instead of "Experiment"? Experiment implies an isolated temporary thing, not an evolving feature that may or may not stay. GPT agrees with me.

[dilirity]

@haqadn oops, I forgot to remove the experiment flag from my wp-config 😅

I also agree with you and chatgpt that it should be experimental. Was debating the two, but I dunno why I left it in as "Experiment".

[haqadn]

LGTM!