Editor: Introduce block context

Backports a new block context feature from Gutenberg. The purpose of this feature is to be able to establish values in a block hierarchy which can be consumed by blocks anywhere lower in the same hierarchy. These values can be established either by the framework, or by other blocks which provide these values. See documentation: https://github.com/WordPress/gutenberg/blob/master/docs/designers-developers/developers/block-api/block-context.md

Props aduth, epiqueras.
Fixes #49927.


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