Align placeholder and iIllustration states between Cover and Image blocks

[rohitmathur-7]

What?

Fixes: #66768

Why?

• Add functionality in Cover Block for displaying the placeholder state only when selected, and the illustration state when not selected—like the Image block does so that we have consistent User Experience.

How?

• Add Placeholder in MediaPlaceholder in Cover Block.

Testing Instructions

• Add Cover Block in the editor.
• Select the Cover block then we should see the placeholder state, and when Cover block is not selected then we should see the illustration state.

Screenshots or screencast

When Cover Block is Selected (Placeholder state):

When Cover Block is not Selected (Illustration State):

[github-actions]

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