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.
git-svn-id: https://develop.svn.wordpress.org/trunk@47265 602fd350-edb4-49c9-b593-d223f7449a82
b6dcb779ed