REST API: Persist attributes for attachment links

In [43437], the link definition implementation caused attachment links to be unexpectedly nested under an attributes key. This changeset restores the prior behavior.

Merges [43681] to trunk.

Props TimothyBlynJacobs, danielbachhuber.
Fixes #44750.

Built from https://develop.svn.wordpress.org/trunk@43973


git-svn-id: https://core.svn.wordpress.org/trunk@43805 1a063a9b-81f0-0310-95a4-ce76da25c4cd