Add json property type with simple validation

[EdColeman]

Adds a JSON PropertyType and minimal validation that verifies the property value can be parsed into a JSON object.

This was inspired by issue #3909, but does not directly address it.

1. Some current properties defer validation until use and this PR does not address that. (That could be done in a follow-on PR)
2. Some configurations require multiple, independent properties be set for a valid configuration, This PR does not attempt to perform that validation - that responsibility is deferred to the implementations that use those properties.

This implementation was to provide a minimal check, that follow on PRs could extend / leverage to introduce additional property validation.

[ddanielr]

Overall changes look good. Minor feedback on some tests.

I like jackson's features over GSON and it might be a good change to fully switch over so there's a single JSON parser used in the project.

[ddanielr]

This question is probably out of scope of the "simple validation" of this PR.

Is there a way to test setting a custom, user-defined property (created under the general.custom prefix that's of type JSON with the shell?

[EdColeman]

As it is now, general.custom is a prefix and does not specify a type.

I think it would take something like a new PREFIX type, maybe something like general.custom.json that could then be used to trigger validation. There may be issues with precedence with general.custom that would short circuit general.custom.json.

Certainly something to discuss as a follow-on.