When requesting a produce for multiple targets the result will be a
dictionary mapping "<target>:<result>". This also changes the GraphQL
API where a json-serialized string is returned.
Splits the module in 2:
* static_handlers: as the name suggests this is where handlers for
static endpoints (e.g. not dependent on the pipeline) are implemented
* schema_generator: this is where the schema is generated from a
manager's pipeline definition, together with the handler for the
autogenerated endpoint