Offline Mode: Switch to local feature flag

[momo-ozawa]

Description

• Switches syncPublishing flag to a local feature flag
• Adds FeatureFlagRolloutStore to facilitate rolling out the flag to a specified percentage of users

How to test

• Change FeatureFlag.swift L45 to return FeatureFlagRolloutStore().isRolloutEnabled(for: self) in order to test the percentage rollout mechanism
• Set a breakpoint on L43 of FeatureFlagRolloutStore.swift
• Run the app with the syncPublishing rollout percentage set to 1%
• ✅ The sync publishing feature flag should disabled (highly likely, but not impossible that it's enabled if you make it into the 1% rollout group)
• When the app hits the breakpoint, note the assigned rollout group and release the breakpoint
• Run the app with the syncPublishing rollout percentage set to a value higher than assigned rollout group / 10 (e.g. if the assigned group was 120, set the rollout percentage to 13)
• ✅ The sync publishing feature flag should enabled

Regression Notes

1. Potential unintended areas of impact sync publishing feature flag

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

3. What automated tests I added (or what prevented me from doing so) none

PR submission checklist:

• [x] I have completed the Regression Notes.
• [x] I have considered adding unit tests for my changes.
• [x] I have considered adding accessibility improvements for my changes.
• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[dangermattic]

	2 Warnings
:warning:	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.
:warning:	This PR is assigned to the milestone 24.8 ❄️. 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 :no_entry_sign: Danger

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23105-6c81ca8
	Version	24.7
	Bundle ID	org.wordpress.alpha
	Commit	6c81ca85db0c8c5bebec31e4a50b7b8e58460f96
	App Center Build	WPiOS - One-Offs #9712

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[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23105-6c81ca8
	Version	24.7
	Bundle ID	com.jetpack.alpha
	Commit	6c81ca85db0c8c5bebec31e4a50b7b8e58460f96
	App Center Build	jetpack-installable-builds #8756

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[dvdchr]

Hi @momo-ozawa 👋 , I'm bumping this PR's milestone to 24.9 since I'm starting code freeze. Feel free to re-target this to the release branch if this is a blocker or intended for 24.8.

[momo-ozawa]

Thanks @dvdchr, I've merged this into release/24.8