common: Add a new StreamUtils class

common: Add a new StreamUtils class

We often use Iterable in our APIs, due its minimal behavior
exposition. Unfortunately, there is no Iterable.stream() method,
the official way is done using StreamSupport and the corresponding
spliterator.

Since this is a very commonly used pattern, we can introduce a
small utility method that will make it less verbose.

At the same time, we can move the StreamFlattener into the new
utils class.

Change-Id: Ia4fed4f8a5da56f08ac133eb10efae970004e511
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/68563
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>