Add a `label` field in the search controller to return localized type for each item

[petitphp]

This change is related to a PR in gutenberg to display localize name for suggestions in the link component.

This change felt too complex to be added to the gutenberg repository and backported afterward. Also I wanted to be able to update existing tests for the REST search handler.

This PR contain two changes :

• Add a new label property in the WP_REST_Search_Controller and update existing handlers to fill this property accordingly
• Add a new WP_REST_Media_Search_Handler handler to uniformize the shape of the data send back by the search endpoint between all types.

Trac ticket: https://core.trac.wordpress.org/ticket/61254

[github-actions[bot]]

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[TimothyBJacobs]

It looks like the associated Gutenberg PR hasn't been merged yet. I'm leaving feedback there for now.