REST API: Introduce rest_{$this->post_type}_item_schema filter to enable...

REST API: Introduce rest_{$this->post_type}_item_schema filter to enable manipulation of schema values.

register_rest_field can be used to add properties to a schema, but no mechanism existed to alter existing properties like "content".
Running the schema through this filter lets plugins append additional sub-properties to existing schema definitions.

Props luisherranz, TimothyBlynJacobs, swissspidy, westonruter, kadamwhite.
Fixes #47779.


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git-svn-id: https://core.svn.wordpress.org/trunk@47065 1a063a9b-81f0-0310-95a4-ce76da25c4cd