REST API: Move object type-specific metadata integrations from the wrapper...

REST API: Move object type-specific metadata integrations from the wrapper functions to the low-level Meta API functions.

Object type-specific actions that should happen before or after modification of metadata have so far been part of the respective wrapper functions. By using action and filter hooks, this changeset ensures they are always executed, even when calling the lower-level Meta API functions directly, which the REST API does as a prime example.

Merges [43729] to trunk.

Props flixos90, spacedmonkey.
Fixes #44467.

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git-svn-id: https://core.svn.wordpress.org/trunk@43814 1a063a9b-81f0-0310-95a4-ce76da25c4cd