My Jetpack: Connect Boost card with the interstitial

[retrofox]

This PR register a new /add-boost route tied to the <BoostInterstitial /> component. Also, Adds a handler to the onAdd() BoostCard property to lead the user to the interstitial page by clicking on the Add Boost link

Note: We are going to add the Go back link in a follow-up.

Fixes #22655

Changes proposed in this Pull Request:

• My Jetpack: Connect Boost card with the interstitial page via /add-boost root

Jetpack product discussion

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

No

Testing instructions:

• Go to My Jetpack
• Set up in order to do not have Boost available in your dashboard. You can hardcode the get_status() method forcing this plugin_absent status.
• Confirm you see the interstitial page by clicking on Add Boost link.

Screen.Recording.2022-02-04.at.6.49.26.AM.mov

[github-actions]

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[oskosk]

Tests well. Tested on JN

[oskosk]

Just pushed a rebase (merge from master)