REST API: Introduce selective link embedding.

Previously the _embed flag would embed all embeddable links in a response even if only a subset of the links were necessary. Now, a list of link relations can be passed in the _embed parameter to restrict the list of embedded objects.

Props rheinardkorf, adamsilverstein, jnylen0, cklosows, chrisvanpatten, TimothyBlynJacobs.
Fixes #39696.

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