Default resolveReference with federated schema

Similar to the way a default `fieldResolver` function can be defined when creating a `new Apollo Server` it would be useful to be able to define a default `__resolveReference` function when using a federated schema.

A use case would be when wanting to resolve multiple references using the same function:

```js
const resolvers = {
Post: {
__resolveReference(reference, context) {
return context.models[reference.__typename].fetchItem(reference.id);
}
},
Comment: {
__resolveReference(reference, context) {
return context.models[reference.__typename].fetchItem(reference.id);
}
},
Review: {
__resolveReference(reference, context) {
return context.models[reference.__typename].fetchItem(reference.id);
}
}
};
```

Of course, one could simply create and use shared function…

```js
function __resolveReference(reference, context) {
return context.models[reference.__typename].fetchItem(reference.id);
}

const resolvers = {
Post: {
__resolveReference
},
Comment: {
__resolveReference
},
Review: {
__resolveReference
}
};
```

…but still, this is quite verbose compared to being able to define a default.

The current federated schema builder uses a default function if no `resolveReference` is defined for an entity:
https://github.com/apollographql/apollo-server/blob/a28d0ac08b47cd79926f7a82a9599a0b9d0b0f10/packages/apollo-federation/src/types.ts#L91-L95

The aim would be to allow for customising this default, maybe by passing one to `buildFederatedSchema`? For example…

```js
// My own default resolveReference function
function resolveReference(reference, context, info) {
return context.models[reference.__typename].fetchItem(reference.id);
}

const schema = buildFederatedSchema({ modules, resolveReference });
```

Start by reading packages/apollo-federation/src/types.ts around lines 91-95 and trace how buildFederatedSchema supplies the current default resolveReference behavior. The change is complete when buildFederatedSchema can accept a configurable default while an entity-specific __resolveReference still takes precedence.

Written by the indexing model from the issue text.