Add a way to retain source file mapping information that can be used post-reification

[grahamboree]

It's often useful to have a data verification step that checks the config values supplied by a designer. This might check that values are within a certain range, or are cross-referenced in another place. e.g. a "texture" field on a game object description might verified by checking for a matching texture file in the project.

Unfortunately, when this verification happens, we don't have any source file mapping for object fields. This means verification can't generate errors that contain the source of the error, so it's difficult to track down and fix issues. The ideal error message syntax would be something like

verification error: cannot find texture named 'myTexture.png'
file: myGameObject.yaml
line: 12

This is a feature that's super useful in development and isn't useful in production, so incurring up to a moderate performance overhead for this is likely worth the tradeoff...provided there's a way to completely disable it in production builds.

Proposed solutions:

• Dynamically add an annotation to the fields of an object at runtime that tracks the file and line number the data originated from. Would need to handle cases where objects are read manually with a FromDoc.
• Add a "SourceFileMapping" member to the classes we care about having source file mappings for. The Reifier then sets the values on this member to indicate the source file and line numbers.

[francoisvn]

Done this downstream with a new attribute ConfigSourceInformation that will auto-populate a string field/property with this info. This can be compiled out with #if so it's not in production builds. Will upstream this soon with a PR :)