Jetpack Focus: Add convenience method for retrieving the removal deadline

[hassaanelgarem]

Part of #19593

Description

This PR adds a convenience function for retrieving the removal deadline remotely.
This will be used in #19593, #19451, and #19449.

Testing Instructions

Ensure that the unit tests are passing.

Regression Notes

1. Potential unintended areas of impact
   Should be none.

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)
   Added a couple of tests to JetpackFeaturesRemovalCoordinatorTests to cover the new function.

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.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19686-c2a5c5f on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19686-c2a5c5f on your iPhone

If you need access to App Center, please ask a maintainer to add you.