Allow to set FetchResult generic type in MockedResponse

[niekert]

Currently there is no way to specify the shape of the FetchResult of a MockedResponse. Because of this, it's easy to pass the incorrect data to MockedResponse. By adding the TData generic to FetchResult it's possible to specify the shape of data that the query should return.

interface Result {
  someMutation: {
    ok: boolean
  }
}

const mockedResponse: MockedResponse<Result> = {
  request: {
    query: SomeDocument
  },
  data: {
    someMutation: {
       ok: true // This is now typechecked
       something: '', // TS error because this does not exist in `Result` type.
    }
  }
}

[apollo-cla]

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