Add merge function that shows which graph results come from

[t92549]

Describe the new feature you'd like
There should be a federated merge function added so that when I perform a GetElements operation, I can see which elements came from which subgraph.

Describe alternatives you've considered
It could be implemented as an option rather than a merge function.

Additional context
Results from ConcatenateMergeFunction look like this:
[el1, el2, el3]
Results from this new merge function would like something like:
{graph1: [el1, el2], graph2: [el3]}

[tb06904]

Might be worth looking at using some part of the stream library to do this as the map() method might work quite well, there is also #3075 open to investigate other features we could use too.