Block Editor: Add helper functions to collect meta box information.

To allow the block editor to render meta boxes, it needs to collect information about how those meta boxes are registered, and format it for the block editor to make use of.

Merges [43778] from the 5.0 branch to trunk.

See #45112.


Built from https://develop.svn.wordpress.org/trunk@44131


git-svn-id: https://core.svn.wordpress.org/trunk@43961 1a063a9b-81f0-0310-95a4-ce76da25c4cd