MULE-7388: Support for xxx-refs in DataSense Retrieval

Endpoints can have transformers-refs and responseTransformer-refs for example. The global elements referenced by them (a list separated by spaces of transformers) should exist as part of the datasense mule configuration or if not startup fails. Added support for *-refs attributes and had to modify logic to support multiple schemaLocations (right now these were overwritten by the last element to be processed)