REST API: Support request level validation

[TimothyBlynJacobs]

Splitting this off from #50244.

We could also support a validate_callback in the register_rest_route on the same level as callback and permission_callback that would take a WP_REST_Request object and do validation for it. This would be helpful for cases where validation has to be done with all the fields together as context, not just individual field validation. We'd do the parameter validation first, and then call validate_callback on the whole request.

Trac ticket: https://core.trac.wordpress.org/ticket/51255#ticket

[TimothyBlynJacobs]

In 48945:

REST API: Support a route-level validation callback.

Most request data is validated on a per-parameter basis. Often, however, additional validation is needed that operates on the entire request object. Currently, this is done in the route callback and often in the prepare_item_for_database method specifically.

#50244 aims to introduce batch processing in the REST API. An important feature is the ability to enforce that all requests have valid data before executing the route callbacks in "pre-validate" mode.

This patch introduces support for calling a validate_callback after all parameter validation has succeeded. That allows moving more validation outside of the route callback and into WP_REST_Request which will improve "pre-validate" support.

Props TimothyBlynJacobs, zieladam.
Fixes #51255.
See #50244.