[XML Markers] Internal Protobuf Schema

[AsherGlick]

A protobuf schema needs to be written to handle the internal filetypes.

It is possible it can be auto generated from the docs, though there might be some file types that are difficult to store. Those custom file types may need to be revisited to see if they can be dynamic instead. Additionally if generated automatically then a mapping between internal types and protobuf types would need to exist somewhere.

Another option, and the suggested one, is to write the schema manually and rely on C++ compiler errors to validate the schema complies with all of the fields required. This will allow custom data types to implement their own serializer functions without needing tooling for bespoke automatic generation.