Add `Optional<JsonNode> JsonNode.asOptional()` convenience method

[cowtowncoder]

(note: part of https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-3 changes)

Currently there is concept of "missing" node (impl MissingNode, detectable via JsonNode.isMissingNode()), returned in cases where Java 8 Optional.empty() would commonly be used. Partly this is due to "missing node" concept predating Jackson using Java 8 as base line, and partly due to wanting to avoid extra layering for common case of "not-missing" nodes.
Be that as it may, use of Optional is convenient with Java 8 Stream methods, so it makes sense to add some support.

To this end, let's add method

Optional<JsonNode> asOptional();

which will return

• Optional.empty() for MissingNode
• Optional.of(this) for all other JsonNode types
• that is, Optional.ofNullable(this.isMissingNode() ? null : this) if implemented at JsonNode level