Apply schema manipulation on input field definitions

What problem does this feature proposal attempt to solve?

Lighthouse applies a process we call schema manipulation for some directives, allowing them to transform the GraphQL AST: https://github.com/nuwave/lighthouse/blob/301930b356faa63da681d564cbef41c7d312f2aa/src/Schema/AST/ASTBuilder.php#L133-L137

The directives @whereConditions and @orderBy implement the ArgManipulator interface. This is only called when they are defined directly on the args of a field.

While there is a directive interface for definitions that are directly applied on types (e.g. input Foo @example), we currently have no mechanism for traversing down into the fields of input types and apply transformation there.

Which possible solutions should be considered?

An implementation for this would entail:

• Adding a new interface InputFieldManipulator
• Add a method to ASTBuilder that applies it to input fields
• Implement the interface for the ArgManipulator directives where it makes sense

Start by reading ASTBuilder.php around the existing schema-manipulation logic and the ArgManipulator contract. Trace how @whereConditions and @orderBy are handled when placed on field arguments, then determine how equivalent traversal should reach input fields. Done means the new InputFieldManipulator mechanism exists and applicable ArgManipulator directives use it where appropriate.