Override the types of input object custom scalar fields

[saga-dasgupta]

query takes custom_scalar_overrides to give a custom scalar in a query's output a specific Rust type. Input objects have no equivalent, so a field typed with a catch-all scalar such as JSON can only be the scalar's type, usually an untyped value.

This adds the same option to typegen, for custom scalar fields of input objects:

#[typegen([
    scalar MyScalar
    input MyInput { myScalar: MyScalar, myScalarList: [MyScalar!]! }
    ...
], custom_scalar_overrides = {
    "MyInput.myScalar" => ::std::primitive::i32,
    "MyInput.myScalarList" => ::std::primitive::i32,
})]

• Paths are "InputObject.field", and the field's base type must be a custom scalar. Lists and nullability are kept, so myScalar above becomes Option<i32> and myScalarList becomes Vec<i32>. oneOf input objects are covered too, for their variants.
• Types not starting with :: are relative to the schema module, as with query.
• Borrowing: an override that borrows gives the input object an 'a lifetime, and needs borrow = true, as with query.
• Validation: it's a typegen error when a path isn't a custom scalar field of an input object, or when an override borrows without borrow = true.
• Shared code: CustomScalarOverride gets parse_all and validate_all, which both options now use. The query option's behaviour and error messages don't change.

Compatibility

Nothing that compiles on main changes: the option is new, and without it the generated types are the same.

[saga-dasgupta]

Moved to #148, from a branch in this repo so that #149 can be stacked on it.