Implemented JsonNode#optional(...) methods

[burdoto]

Closes #2145, though it might be wanted to add the methods #optional(String) and #optional(int) to the TreeNode in jackson-core.
If that is the case, let me know, and I will open PRs in both repositories.
This implementation is very rudimentary and uses the #path(...) methods, while checking with #isMissingNode() whether an empty optional should be returned.

[burdoto]

Is this still a thing?

[cowtowncoder]

@burdoto Apologies for missing this PR. I am actually thinking of this as part of bigger overhaul, listed on

https://github.com/FasterXML/jackson-future-ideas/wiki/Jackson3-Changes---JsonNode

to be converted into

https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3

and related discussion under

FasterXML/jackson-future-ideas#36

(I am trying to figure a way to define light-weight specifications for larger changes)

[cowtowncoder]

So: although I think this is a good idea in general, I think I'll do this as part of bigger set of changes described here:

FasterXML/jackson-future-ideas#36

So, thank you for contributing this; it will get in 3.0 in some form.