Support `@required` and `@optional` directives

### Is your feature request related to a problem? Please describe.

I'd like to add built-in support for _any_ method that could generate the following TypeScript types from a schema for use by a client:

```ts
export type MyInput = {
a?: InputMaybe;
b?: Scalars['String']['input'];
c: InputMaybe;
d: Scalars['String']['input'];
};
```

### Describe the solution you'd like

The best proposal I've seen uses `@required` and `@optional` input directives, as described in https://github.com/graphql/graphql-spec/issues/872:

```graphql
"""
This input is optional, not nullable.
If the client insists on sending an explicit null value, the behavior is undefined.
"""
directive @optional on ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION

"""
This input is nullable, not optional.
If the client insists on omitting the input value, the behavior is undefined.
"""
directive @required on ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION

input MyInput {
a: String
b: String @optional
c: String @required
d: String!
}
```

This addresses a [very common concern](https://github.com/graphql/graphql-spec/issues/476), and since it only uses directives, it's completely forward- and backward-compatible and doesn't require any changes to the GraphQL spec.

My suggestion is to support the above with a new TypeScript plugin option:

```ts
export default {
generates: {
"path/to/file": {
config: {
directiveArgumentAndInputFieldNarrowing: {
nonNullable: "optional",
nonOptional: "required",
},
},
},
},
} satisfies CodegenConfig;
```

### Describe alternatives you've considered

_No response_

### Any additional important details?

If we could agree on the right approach, I'd be happy to try implementing this myself.