REST API: Include `block_version` on Post `content` object.

The `block_version` denotes which version of Blocks the `post_content` contains. Introduces new `block_version()` function for versioning Blocks.

Merges [43770] from the 5.0 branch to trunk.

Props danielbachhuber, birgire.
Fixes #43887.

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git-svn-id: https://core.svn.wordpress.org/trunk@43957 1a063a9b-81f0-0310-95a4-ce76da25c4cd