Create a tool that can generate C/C++ data models from Ion Schema

[popematt]

This is similar to amazon-ion/ion-schema-kotlin#146, but the generated classes (or structs) should be for use with C/C++ (and possibly Rust).

• This does not necessarily require a C/C++ implementation of Ion Schema before it can be done. (There is no reason why a JVM-based tool couldn't generate code for a different language.)
• We should see if this can be solved in a general way—it might be possible to leverage something like Kotlin Multiplatform so that we can generate code for Kotlin data classes that can be compiled for use with native, jvm, or javascript code.