REST API: Accept POST requests in the block renderer endpoint.

Rendering a block is idempotent, so a GET is the most natural request method. However, the maximum length of URLs prevented blocks with large attributes from being rendered.

Props ryankienstra.
Fixes #49680.

Built from https://develop.svn.wordpress.org/trunk@47756


git-svn-id: https://core.svn.wordpress.org/trunk@47532 1a063a9b-81f0-0310-95a4-ce76da25c4cd