Allow multiple targets with targetType on @Query & @Mutation

| Q                | A
| ---------------- | -----
| Bug report?      | no
| Feature request? | yes
| BC Break report? | no
| Version/Branch   | 1.0.0

When using multiple schemas, it can be convenient to be able to export a query or mutation on multiple targets. 
Attribute `targetType` on `@Query` and `@Mutation` should allow to specify multiple target. 

