Add a method to RestTestClient API that provides a JsonContentAssert

[donalmurtagh]

See issue #35624

[vpelikh]

I very much miss JsonContent assertions, and this is an awesome change. @donalmurtagh, while you are on it, can you also add the same method to WebTestClient? I think the reactive and imperative test clients should have symmetrical API.

[donalmurtagh]

I very much miss JsonContent assertions

You can still use them (without this new method). The "Current Workaround" section of #35624 shows how.

can you also add the same method to WebTestClient?

The issue I raised is specifically about adding this method to RestTestClient. I'd rather wait and see whether this PR is accepted before adding a similar method to WebTestClient.

I think the reactive and imperative test clients should have symmetrical API.

If that's supposed to be the case, they should implement a common interface, but they don't.