[Site Editor] Add remote FF to SiteEditorMVP and default to on

[thomashorta]

Fixes #18441

Add remote field to Site Editor MVP FF (site_editor_mvp) and default it to ON.

Not ready for merge: we probably need to add something to the release notes.

To test:
Verify that on a clean install, the Site Editor MVP feature (virtual homepage item for block-based sites in the Pages List) is on by default. Also, check that the site_editor_mvp is available in the Debug Settings.

Instructions to test the feature itself can be found here: #18436

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.
• N/A UI Changes testing checklist

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr18454-55d5762
	Commit	55d5762
	Direct Download	wordpress-prototype-build-pr18454-55d5762.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr18454-55d5762
	Commit	55d5762
	Direct Download	jetpack-prototype-build-pr18454-55d5762.apk

Note: Google Login is not supported on these builds.

[RenanLukas]

LGTM