REST API: Add uniqueItems as a permitted endpoint argument JSON Schema keyword.

Support for the `uniqueItems` keyword was added in [48357], but wasn't included in the list of permitted keywords for `WP_REST_Controller::get_endpoint_args_for_item_schema()`. This prevented the keyword from being automatically supported if specified in a controller's item schema.

Props TimothyBlynJacobs.
Merges [48796] to the 5.5 branch.
Fixes #51021.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48569 1a063a9b-81f0-0310-95a4-ce76da25c4cd