DATAES-77 - Search across different indices

Add test to demonstrate searching into heterogeneous indexes and building entities of different types. In a nutshall, the responsiblity lies with the client, spring data elasticsearch cannot and should not try to discover entity types, as that would leading building an object composition logic which is not the core area of this library.

With this test, we demonstrate the usage of SearchResultMapper to compose an aggregator object that holds properties from two dfferent entities, The Aggregator is declared as a document but is not indexes (This is not ideal) and we don't recommend to declare @Documents that are not indexed. Please use this as just a pointer to implement SearchResultMapper, this demostartion is not meant to be duplicated in your production systems. There are better ways to compose hetro objects from multi index searches.