Add "required" methods in `JsonNode`: `required(String | int)`, `requiredAt(JsonPointer)`

[cowtowncoder]

As per ideas on https://github.com/FasterXML/jackson-future-ideas/wiki/Jackson3-Changes---JsonNode it seems we could add couple of new, useful "validating" accessor methods, similar to how many other Java JSON libraries mandate presence of value when using accessors.

Here's a suggested list:

• required(locator): similar to get(locator) and path(locator) but will throw exception (instead of returning null or MissingNode) with message indicating locator
  • locator is either property name (for Objects) or int index (for Arrays), same as with other accessors
  • Returns JsonNode at location, guaranteed not to be null or MissingNode
  • Exception to throw needs to be unchecked exception, even for 2.10 (in 3.0 we will likely change all Jackson exceptions to be unchecked) -- partly since we do not have access to context with any of the accessors
• requiredAt(jsonPointer) (either String or pre-compiled JsonPointer)
  • same as above, but allows convenient single-call verificatiion
• require(): verify that node is NOT missingNode; if it is, throw same exception as previous accessors would
  • essentially, root.required("foo") would be equivalent to root.path("foo").require() (except for slightly different error message)
  • returns this
• requireNonNull(): similarly to require(), but also throw exception if "this" is NullNode (returns true for isNull())

So: these could improve JsonNode ergonomics a lot.

There are obviously many more we could consider (for example, whole set of requireInt() or such), but I think incremental improvements are a good way to go here.

[cowtowncoder]

Methods added as suggested: for 2.x (at least), failure indicated by IllegalArgumentException: for 3.x will change to new unchecked Jackson-specific exceptions.