Posts, Post Types: Add labels for post transformation messages.

Because the existing `post_updated_messages` filter can be modified dynamically based on post state, it's unreliable to use with REST API clients. Instead, these new labels give clients stateless equivalents.

Merges [43744] from the 5.0 branch to trunk.

Props earnjam.
Fixes #45101.

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git-svn-id: https://core.svn.wordpress.org/trunk@43940 1a063a9b-81f0-0310-95a4-ce76da25c4cd