data-stream

We should add a Github action that runs make generate and succeeds if there is no diff. It should run on each push to a PR.

We could do the same with proto files (generate them and make sure there's no diff).

sbt experiment command in root should execute all experiments (c.f. flex.experiment package). However, for now, only one experiment is executed (with arg0). Therefore, the experiment command that does not have an argument must perform all the experiment codes. See Tasks.

Feature description

Introduce DataStream.flatten() method similar to v4:

A shorthand for streams of arrays or iterables to flatten them.

More efficient equivalent of: .flatmap(i => i); Works on streams of async iterables too.

**If