Spacer: Add border support

[karthick-murugan]

What?

Add border block support to the Spacer block.

Part of #43247

Why?

Spacer block is missing border support.

How?

Adds the border block support in block.json

Testing Instructions

• Go to Global Styles setting ( under appearance > editor > styles > edit styles > blocks )
• Make sure that Spacer block's border is configurable via Global Styles
• Verify that Global Styles are applied correctly in the editor and frontend
• Edit template/page, Add Spacer block and Apply the border styles
• Verify that block styles take precedence over global styles
• Verify that block borders display correctly in both the editor and frontend

Testing Instructions for Keyboard

None

Screenshots or screencast

In Backend:

In Frontend:

[github-actions]

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[github-actions]

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[aaronrobertshaw]

Thanks for proposing this PR @karthick-murugan 👍

I'm not sure adding visual styles to the Spacer block is the direction we want to go. My understanding is this block is meant purely for layout and spacing purposes.

The same concern is raised over on the PR proposing to add color support to the Spacer block (#66433).

I've added the Needs Design Feedback label in order to get a design perspective on this.