Add support for loading and interacting with GeoJSON data in Blueprint

[azrogers]

The companion to CesiumGS/cesium-native#1154. This PR adds blueprint functions for loading and interacting with GeoJSON data. In particular, it provides the ability to load GeoJSON from a string or from Cesium ion, the ability to query nodes, and the ability to obtain primitives as values usable in Unreal.

One note is that access to the properties of a vector node requires blueprint functionality for interacting with JSON objects. I elected to push this responsibility onto the JSON Blueprint Utilities plugin, which is an official Unreal plugin but not enabled by default. Returning the FJsonObjectWrapper struct from a method is supported without this plugin enabled, but users will have to enable that plugin if they want to be able to do anything with it.

[azrogers]

@kring @j9liu I think that's all the review comments taken care of, minus the #include question (are we committing to angle bracket includes in Unreal)?

[kring]

Looks like we still have some VectorDocuments:

[kring]

There's a lot of "vector document" in a bunch of other places as well, including filenames and parameter names (which matter because they're shown as output in names in Blueprint). Can I bug you to do a pass over the code to replace all those occurrences, and then I'll look for any that might have been missed while I'm reviewing?

[azrogers]

@kring I believe I've caught all of them, let me know if you find any others.

[kring]

Thanks @azrogers!