Add JSON support to the REST API

AFAIK the REST API only supports application/x-www-form-urlencoded content type.
Adding support for JSON content type would be a nice upgrade.

Devs are used to POST json, not form-like data.
Having to convert booleans into a string (or sometimes an integer, see MB-59382) is always a surprise.

Moreover, having a JSON-based API would unlock a better design with the possibility to pass complex objects instead of having prefixed properties, i.e : historyRetentionCollectionDefault, historyRetentionBytes, historyRetentionSeconds. And I picked the history related properties for the sake of brevity, if you look at the compaction related properties...

But let's start small and add JSON support to the current design.