Roadmap to 1.0

[LucioFranco]

This is the roadmap to 1.0 for prost defining the scope of the project.

• Make prost-build modular
• Add Json support
• Unknown fields
• Extensions
• Better Any support
• Add support for Arc/Rc/Box, Ref Support for std::sync::Arc as indirection #429

cc @danburkert @neoeinstein