fix: return proper JSON from REST API

[joske]

Motivation

Some endpoints return just a Display implementation for serializing of a Value. This leads to invalid JSON. See #3361. Note that this approach does preserve order.

Filing as draft to solicit feedback.

Test Plan

Tested manually as described in the linked issue.

[joske]

@vicsn @niklaslong I think this approach is acceptable, it only changes the behaviour for the REST call.

[kaimast]

Closing this in favor of #3751.