WP 4.6 integration with register_meta()

[sc0ttkclark]

We should start utilizing register_meta() if WP 4.6+ to register our meta keys for each content type. Same way we do register_post_type() (cached info to make it easy to run the registers without a bunch of requests).

We can enable show_in_rest if REST is enabled for that field, but we'll still need to keep the register_rest_field() calls in place until the WP REST API merges their new meta endpoint(s) into core and that function is no longer needed (likely a WP 4.7+ check).

[sc0ttkclark]

Need to add the code here:
https://github.com/pods-framework/pods/blob/release/2.7/classes/PodsRESTFields.php#L131

[sc0ttkclark]

I want to do this now but I feel like it might be too heavy to run this for every single pod field (for a WP object) prior to 2.8's PodsObject

[sc0ttkclark]

To further explain, if we just run it in PodsRESTFields, it won't hit all fields because it would only register for fields that a user has access to or fields that have read/write set on them for REST purposes (entirely separate context from meta which can be used apart from the REST API)

[quasel]

Link from #4096 -> https://core.trac.wordpress.org/ticket/40318