Field extensions

Not being able to add directives to an existing field definition has been a growing pain point in Apollo Kotlin. This RFC solves that.

* [~Spec edits~](https://github.com/graphql/graphql-spec/pull/1196) (closed)
* [GAP-4](https://github.com/graphql/gaps/pull/4/)
* [graphql-js PR](https://github.com/graphql/graphql-js/pull/4522)
* [apollo-kotlin PR](https://github.com/apollographql/apollo-kotlin/pull/6856)

See https://github.com/graphql/graphql-spec/issues/952 for the initial use case

## Current status

Object type extensions may extend existing fields.

In this example, we deprecate the field `id` on type `Query` by adding a
`@deprecated` directive:

```graphql example
type Query {
id: String
}
```

```graphql example
extend type Query {
id: String @deprecated(reason: "Use newId")
}
```

## Open questions

* Are extensions a merge? Or just additive?