add propertyNamesSpliterator

[pjfanning]

will likely break downstream Jackson projects until we implement the equivalent method

[JooHyukKim]

Is this part of any JSTEP? If not I think we can add somewhere in JSTEP for 2->3 migration doc later on.

[cowtowncoder]

Is this part of any JSTEP? If not I think we can add somewhere in JSTEP for 2->3 migration doc later on.

Would belong under https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3 and I think @pjfanning probably noticed my adding #4863 (sort of related). TreeNode additions are bit limited due to Java limitation on co-variance (cannot add return value of Stream<TreeNode> because not legal to override with Stream<JsonNode>) but can do this for property names (since keys are still Strings).

[cowtowncoder]

@pjfanning Quick question on need/rationale here: I forget details of how Spliterators are useful (as building blocks for Streams... ? ). So would this be useful for actual use cases?

Another question is if this should go in 2.19 instead -- I plan on adding most JsonNode additions, where possible, in 2.19 not just 3.0 (but some will of course need to go there).

[pjfanning]

I saw the changes for stream support and spliterators are most used as an iterator that can be split and processed in parallel (eg parallel streams).
The changes don't look very drastic so I'm happy to not merge the changes and wait to see if there is explicit demand for this. What I mean is that changes like this could be added at any time if needed and they don't really need a major release.