Extending types without @key directive

**Package Version** - "@apollo/federation": "0.22.0"

**TL;DR**
Is it possible to extend types that doesn't need keys and federation would know where to send requested fields to in parallel (as with general Query type)?

Given a schemas of 2 services: `products` and `reviews`. When federation merges these 2 schemas, no validation errors returned and schemas are composed successfully. The key type to look at is `ProductsMutation`, defined in **Products** and extended by **Reviews**.

**Schema of `products` service**
```graphql
extend type Query {
products(limit: Int): [Product!]
product(id: ID!): Product
}

extend type Mutation {
products: ProductsMutation
}

# namespaced mutation
type ProductsMutation {
save(data: NewProductData!): SaveProductResult
}

type SaveProductResult {
product: Product
error: Error
}

input NewProductData {
name: String!
}

type Product @key(fields: "id") {
id: ID!
url: String!
name: String!
}
```

**Schema of `reviews` service**
```graphql
extend type Query {
reviews: [Review!]
}

extend type Product @key(fields: "id") {
id: ID! @external
reviews: [Review!]
}

type Review {
id: ID!
text: String!
}

# extending namespaced mutation
extend type ProductsMutation {
addReview(data: NewReviewData!): ReviewAddedResult
}

input NewReviewData {
text: String!
productId: ID!
}

type ReviewAddedResult {
header: String!
text: String!
actionText: String!
}
```

Given two mutations:
```graphql
mutation CreateProduct($data: NewProductData!) {
products {
save(data: $data) {
product {
name
url
id
}
}
}
}

mutation AddReview($review:NewReviewData!) {
products {
addReview(data:$review) {
text
header
actionText
}
}
}
```

`CreateProduct` is sent to **Products** service, as well as `AddReview` mutation. As **Products** service doesn't know about `addReview` mutation it fails with graphql validation error. Is it expected behaviour? If expected, then why validation does not fail? If unexpected, then, I suppose it's a bug.

Query plan of execution these mutations:
**CreateProduct**
```
QueryPlan {
Fetch(service: "products") {
{
products {
save(data: $data) {
product {
name
url
id
}
}
}
}
},
}
```

**AddReview**
```
QueryPlan {
Fetch(service: "products") {
{
products {
addReview(data: $review) {
text
header
actionText
}
}
}
},
}
```

Start with the products and reviews schemas and compare the composed query plans shown for CreateProduct and AddReview. Trace how the extended ProductsMutation field is validated and assigned to a service; done means the behavior is either corrected with coverage for this schema pair or documented as an intentional limitation.

Written by the indexing model from the issue text.