Improve performance of Cover block processor

[fluiddot]

This PR follows the same approach taken in #22896 to improve the performance of Gutenberg processors. The overall improvement will be less significant in this case (that's why the PR is marked as Enhancement instead of Bug) mainly because the File block is used by fewer users than Image/Gallery blocks. Nevertheless, it will speed up the post-saving process.

To test:

Follow the testing instructions to cover the functionality related to the processors for the block:

• Cover Block:
  • Close/Re-open post with an ongoing image upload
  • Close post with an ongoing image upload

Regression Notes

1. Potential unintended areas of impact
   The block processors are only used when saving a post. No other area should be impacted.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested saving a post with Cover blocks.

3. What automated tests I added (or what prevented me from doing so)
   Unit tests of the Cover block have been updated.

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]

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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	pr22898-67e3041
	Version	24.5
	Bundle ID	org.wordpress.alpha
	Commit	67e3041
	App Center Build	WPiOS - One-Offs #9256

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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	pr22898-67e3041
	Version	24.5
	Bundle ID	com.jetpack.alpha
	Commit	67e3041
	App Center Build	jetpack-installable-builds #8300

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[fluiddot]

While testing this PR, I spotted the issue #22901. I'll take the opportunity to address it here.

[irfano]

I'm moving this to the next milestone since this is not a blocker, and the code freeze will be completed today.

[momo-ozawa]

👋 Hey @fluiddot, I'm bumping this PR to 24.8 since it's code freeze day. If this PR needs to target 24.7, please target the release branch once it's been cut. Thanks!