Takes a payload which is a Collection of Collections and turns into a single List. For example, if the payload is a Collection
which contains a Collection with elements A and B and another Collection with elements C and D, this will turn them into
a single Collection with elements A, B, C and D.
This transformer will also work on MuleMessageCollections. In this case, it will take the individual Collection
payloads of each MuleMessage and merge them into a single Collection on a new MuleMessage.