[schema] Cleanup infer/dontInfer

[freiksenet]

Currently we have non-symmetrical relation between directives @infer/@dontInfer and extensions infer. @infer maps to infer: true, while @dontInfer maps to infer: false. This not only breaks the convention that directives directly map onto extensions, but also the convention that extension values are objects.

I propose the following changes:

1. Add a parameter to @infer - mode. It accepts an enum of ENABLED | DISABLED | DEPRECATED
2. @infer default becomes @infer(mode: 'ENABLED') and { infer: { mode: 'ENABLED' } }.
3. @dontInfer stays the same and becomes { infer: { mode: 'DISABLED } }`.
4. infer: true and infer: false get converted to corresponding object and trigger a deprecation warning.
5. DEPRECATED for infer raises a warning if declared schema doesn't match inferred schema. It is to be used, for example, by core team when we define core types like File or ImageSharp.

CC @vladar @stefanprobst @ChristopherBiscardi

[pieh]

Question about:

infer: true and infer: false get converted to corresponding object and trigger a deprecation warning.

This is for when users/plugins declare type using schema.buildObjectType? If so, should using @infer (without params) and @dontInfer directives using SDL also trigger deprecation warning?

[freiksenet]

This is for when users/plugins declare type using schema.buildObjectType?

Yes, they should do infer: {} or infer: { mode: 'ENABLED' }.

If so, should using @infer (without params) and @dontInfer directives using SDL also trigger deprecation warning?

I think it's fine to have defaults, so @infer is infer: {} that is a valid config.