[feature/zipwith] : implementing ZipWith functionnality and added a test

[nbtk123]

Motivation

• Added ZipWith functionality, e.g. taking elements from several upstreams in a round-robin fashion, bundling them to a single element, then sending it downstream.

Modifications

• Implementation is in utils.go, the test is in flow_test.go.

Verify change

• [V] Make sure the change passes the CI checks.

[reugn]

Hi @nbtk123, thanks for your interest in the project.
I have a question related to the implementation of ZipWith. It will not work with finite streams where the number of elements is different. Also, what would be a real use case for this feature with possibly infinite streams? It does make sense to group elements by the key extracted by a selector function in a window.