Add remaining public APIs for Keystone

[kean]

This PR gets Keystone to compile again (when #24399 and #24404 are also merged). It contains a lot of no-brainer changes, but there are some minor Swift rewrites too (also no-brainer).

To test:

Regression Notes

1. Potential unintended areas of impact

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

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

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	3 Warnings
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
⚠️	This PR is assigned to the milestone 25.9. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by 🚫 Danger

[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
	Configuration	Release-Alpha
	Build Number	27102
	Version	PR #24406
	Bundle ID	com.jetpack.alpha
	Commit	6632d02
	Installation URL	225h8o39hfgh0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[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
	Configuration	Release-Alpha
	Build Number	27102
	Version	PR #24406
	Bundle ID	org.wordpress.alpha
	Commit	6632d02
	Installation URL	3343ju8d5f2ag

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[mokagio]

Unlocked merging to address conflicts with #24378, if any