Adds `t.modify` API for modifying inherited types.
Use cases:
- If you have an `interface` type, but want to override the resolver or description for the field per-`objectType`
- If an inherited field's value is abstract, and you want to narrow it to a subtype which conforms to the inherited type
- If you want to add additional optional arguments for an inherited field, as is permitted by the spec
The types should validate that only fields inherited from interfaces can be modified, and the `type` can only be
changed if the result of that field is an abstract type which implements the existing inherited return type.
Tim Griesser