Spread unions across service boundaries

[AlecAivazis]

We should explore if there is a reasonable story and motivation for allowing different services to contribute different members to the same union.

Related to #58

[AlecAivazis]

If a union is spread across multiple services we can create a step that is inserted based on the value of __typeName. It's important that a node query does not end up at a service that does not have the underlying type defined as that will result in an error