Incomplete type annotation for apollo's KeySpecifier type

### Which packages are impacted by your issue?

@graphql-codegen/typescript-apollo-client-helpers

### Describe the bug

There is a disconnect i.e. a missing type between the type generated for `KeySpecifier` and apollo's [actual interface](https://github.com/apollographql/apollo-client/blob/4e147b579bff210dbbc884d918edb1e69c2cfa9f/src/cache/inmemory/policies.ts#L112).

### Your Example Website or App

https://codesandbox.io/s/bug-gqlgen-apollo-keyspecifier-type-mismatch-wgjgkj?file=/repro.ts

### Steps to Reproduce the Bug or Issue

For example, considering the schema

```gql
type T {
Foo: String
Bar: String
}
```
After running the `typescript-apollo-client-helpers`, plugin, it would generate the following type policy for `T`:

```ts
export type TKeySpecifier = ('Foo' | 'Bar' | TKeySpecifier)[];
export type TFieldPolicy = { /* irrelevant to this issue */ };
```

### Expected behavior

Considering the [actual type shape](https://github.com/apollographql/apollo-client/blob/4e147b579bff210dbbc884d918edb1e69c2cfa9f/src/cache/inmemory/policies.ts#L112) for the `keyFields` property —which includes a function to generate a type's `keyField`— I would expect the following type annotation:

```ts
export type TKeySpecifier = ('Foo' | 'Bar' | TKeySpecifier | (o: Readonly, context: KeyFieldsContext) => string)[];
```

### Screenshots or Videos

_No response_

### Platform

- OS: macOS
- NodeJS: 16.4
- `graphql` version: 15.5.0
- `@graphql-codegen/*` version(s): 2.6.1

### Codegen Config File

_No response_

### Additional context

_No response_