Design and create schema and associated codegen tooling

[iboB]

Project for main issue

Motivation: #240

This task has multiple directions:

• Language - what language should we use to describe the schema
  • C++ discussed here: Redesigning schema generation #148
  • Declarative language discussed here: Schema language #241
• Codegen - we will switch to a declarative plain-text source for schemas and use codegen to create the internal struct serialization. The codegen tool must allow plugins for multiple languages. The core and C++ generator will be hosted here, and generators for other languages in their respective repositories.
  • For C++ this will likely be based on template helpers which can use the generated code to perform i/o
  • C++-only helpers for provider implementation also need to be created
• Distribution - tooling to fetch schemas and integrate them into a build process
• Design - what actually goes into a schema, how it's structured and how it's presented. The meaning of different schema items and how they affect frame io. Discussion here Schema design #242