Blocks: Add context fields to WP_Block_Type

New block context related fields were added as part of https://github.com/WordPress/gutenberg/pull/22686. This changest backports them to WP_Block_Type class.

Props aduth, spacedmonkey, mcsf, epiqueras.
Fixes #47656.  


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git-svn-id: https://core.svn.wordpress.org/trunk@47886 1a063a9b-81f0-0310-95a4-ce76da25c4cd