Spacer: Add example attribute to block

[SainathPoojary]

Part of #64707

What?

This PR adds an example attribute to the Spacer block’s block.json to provide a default height and width in the block’s example configuration.

Testing Instructions

1. Open a post or page in the WordPress block editor.
2. Open the block inserter by clicking the “+” button.
3. Hover over the Spacer block in the inserter.

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: SainathPoojary <[email protected]>
Co-authored-by: hanneslsm <[email protected]>

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[hanneslsm]

Not sure about this one, especially because the info text reads "white space", which now is literally white space.

CC @WordPress/gutenberg-design