Why the type of the link property in ApolloClientConfig is string instead of ApolloLink

### Question

Why the type of the link property in ApolloClientConfig is String instead of ApolloLink

### Description

I want to use my 'authMiddlewareLink' instead of the default link。
But I got these problems from VSCode

> Type 'ApolloLink' is not assignable to type 'string'.

### Code
```
const authMiddlewareLink = new ApolloLink((operation, forward) => {
const token = getUserToken();
const headers = token ? { 'API-TOKEN': token } : {};
operation.setContext({
headers,
});
return forward(operation);
});

export const { apolloClient } = createApolloClient({
link: authMiddlewareLink,
...defaultOptions,
});
```

### Version
0.21.3