Review entire public surface

[brettsam]

Minimize surface area for v1. Make sure that anything referencing GRPC or that couples us to System.Text.Json isn't exposed. Things like Converters can be all internal and exposed later to give us more time for API design.

[fabiocav]

First iteration done here: https://apiview.dev/Assemblies/Review/af54138c8aa0459c8e91df73dfa71e98

[brettsam]

Will do one final review pre-1.0

[fabiocav]

Last round or reviews completed. Added with #262