Support schema without Query type when using federation

[cappuc]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

Changes

When using federation it is possible to have a schema without the Query type and only types extensions.
With this change it is possible to omit the Query type

Breaking changes

none.

[spawnia]

This is not that easy, see graphql/graphql-spec#631

If we make this possible, it should not be limited to Federation. We need to make sure that validation and introspection still work.

[spawnia]

See webonyx/graphql-php#920

[cappuc]

Yes it makes sense to implement it globally but it's more complex because it needs to handle validation and introspection correctly.

With this "federation only" implementation the Query type is always defined with at least _service and _entities queries so the rest of the implementation doesn't need changes.

Anyway I did some tests with apollo gateway and this implementation (only type extension and no root Query) and it seems to works fine