Validation: forbid __schema outside of the root type

[Geal]

According to @IvanGoncharov , __schema should not be used outside of the root type: apollographql/router#5255 (comment)

Do we check this somewhere?

[SimonSapin]

In a typical schema, non-root types don’t have a __schema field so converting AST to an ExecutableDocument would fail with a BuildError::UndefinedField. In code, meta-fields like __schema are not present in the Rust struct ObjecType.fields but are returned if you instead call schema.type_field(type_name, field_name).

Defining a schema like type Something { __schema: Int } is disallowed per spec but we don’t yet implement that validation rule: #743

[SimonSapin]

Closing as duplicate of #743