Allow expressing JSONObject keys dynamically, as well as explicit pair-values

[john-parton]

In working towards supporting ABSENT ON NULL for JSONArray, I did some digging on what it would take to implement it for JSONObject as well, and essentially, because the init signature consumes all the kwargs, there's not really a good way to configure its behavior without conflicting with a key definition.

If you could specify the items of a JSONObject positionally, then you get the aforementioned benefit of being able to have kwargs as well without conflicting, but you also get the added benefit of being able to construct objects with non-static keys.

As an example esoteric usage, if you're trying to get the database to omit valid objects for the Elasticsearch DSL, you would almost certainly need to be able to build objects with non-static keys.

For example ​https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-term-query.html

The key "user.id" refers to a field in elastic.

See some discussion here: ​https://github.com/django/django/pull/19097#issuecomment-2611270246

I'll throw together a patch.

[john-parton]

Pull request/patch here: ​https://github.com/django/django/pull/19101

Not too bad for the actual code change, just a few lines. Several tests added and expanded the Usage section of the documentation sufficiently to cover the various use cases.