REST API: Sanitize block renderer attributes.

In [48069] the Block Renderer was changed to register a single route for all dynamic blocks. Validation was dynamically applied based on the requested block, but sanitization was not. This commit adds the same sanitization back to the block attributes.

Props manooweb.
Fixes #50620. See #48079.

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git-svn-id: https://core.svn.wordpress.org/trunk@48206 1a063a9b-81f0-0310-95a4-ce76da25c4cd