feat(composition): Implement shallow argument addition

[tninesling]

Summary

Ports the logic for Merger.addArgumentsShallow. The port for mergeDefaultValue will be coming in a separate PR.

New HasArguments trait

In the TS implementation, this method is generic over types which are FieldDefinition or DirectiveDefinition. Since we can't create the same sum type in Rust, I abstracted out the various calls to the AST node which get the arguments we're looking for. Then, we can call add_arguments_shallow on any position which implements that contract. This also includes the implementation of that trait for object field, interface field, and directive definition positions.

Argument positions can now .insert()

The aforementioned trait is implemented using new .insert() methods on the various argument types. These automatically insert the referencers required on insert, so our schema metadata is up-to-date.

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