`WP_REST_Block_Renderer_Controller` allows rendering of server-side rendered blocks, whilst `WP_REST_Blocks_Controller` allows retrieving of reusable blocks.
Merges [43805] and [43806] from the 5.0 branch to trunk.
Props desrosj, danielbachhuber, pento, Presskopp, swissspidy.
See #45065, #45098.
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