Clear and concise description of the problem

We have heard from a few places (GraphQL, Azure specs) where it would be useful to explicitly annotate the usage of a data type as being input only, output only, or both. For GraphQL, input and output shapes have different constraints, and so this annotation would allow a graphql validator/emitter to help developers stick to these constraints. For REST APIs, client codegen can be different depending on if a type is input/output/both, and it can be a breaking change to move between these access categories.

There are various options we could land on:

• Syntax, like in/out/inout modifiers on declaration forms.
• Decorator in core, either @input/@output/@inputOutput, or @usage(AccessFlag.Input).
• Language/protocol specific decorators

There may be more options, so let's explore the space and see what makes sense.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.