REST API: Check required properties are provided when validating an object.

Previously, the WP_REST_Request object validated that top-level properties were defined, but this did not extend to those object's required properties. This adds validation to rest_validate_value_from_schema() directly.

Both the v3 and v4 JSON Schema syntax for required properties is supported.

Props sorenbronsted.
Fixes #48818.

Built from https://develop.svn.wordpress.org/trunk@47809


git-svn-id: https://core.svn.wordpress.org/trunk@47585 1a063a9b-81f0-0310-95a4-ce76da25c4cd