Incomplete support for introspection queries

[Geal]

In the selections parsing, we recognize introspection fields using the __ prefix:

router/apollo-router/src/spec/selection.rs

Lines 88 to 111 in 440b59b

	let field_type = if field_name.as_str() == "__typename" {
	FieldType::String
	} else if field_name.starts_with("__") {
	FieldType::Introspection(field_name.clone())
	} else {
	current_type
	.inner_type_name()
	.and_then(|name| {
	//looking into object types
	schema
	.object_types
	.get(name)
	.and_then(|ty| ty.field(&field_name))
	// otherwise, it might be an interface
	.or_else(|| {
	schema
	.interfaces
	.get(name)
	.and_then(|ty| ty.field(&field_name))
	})
	})
	.ok_or_else(|| SpecError::InvalidType(current_type.clone()))?
	.clone()
	};

But it is not enough for introspection queries. With the following query:

query {
  __schema {
    queryType {
      name
    }
  }
}

We would recognize __schema, but then queryType is unknown because we do not know the fields of the __Schema type.

To fix that, the schema object should carry by default the introspection object types

[bnjjj]

We need to support these types https://spec.graphql.org/October2021/#sec-Schema-Introspection.Schema-Introspection-Schema