feat: Add support for for comprehensions.

[He-Pin]

Motivation:
refs: #654

Note:

1. These methods are only added for scaladsl
2. foreach method just work as fs2, where the Stream needs compile aka pekko's run. but different in ZIO, where foreach === runForeach.

Result:
for comprehensions works as fs2 for both Source and Flow in scaladsl

[pjfanning]

I would still like the new methods marked as experimental. I just think there are chances that we ay need to tweak this support if users try out for comprehensions and find some oddities.

[pjfanning]

@pjfanning Do you mean add @ApiMayChange?

Maybe not that one - the API signatures won't change but we may need to tweak the internals of the functions.

Even we add something like this to the Scaladoc for the new functions.

Support for `for` comprehensions is experimental and it is possible that we might need to change the internal implementation to better support it.

[He-Pin]

I think the only in question is foreach, where ZIO alias to runForEach but fs2 alias to foreach and then needs a Stream#compile to run to a F[_], as Pekko stream always needs an explicitly run, so I think we can follow what fs2 does, and that's what @gmethvin once suggested too.

and for the withFilter and flatmMap , I think the currently shape should be fine, but I will update the scaladoc.

[pjfanning]

lgtm - I would like to see some other reviews though

[laglangyue]

LGTM for code.
This is an Amazing feature, we shoudl update the document also,maybe in docs/src/main/paradox/stream/operators/Flow/ .

[jxnu-liguobin]

lgtm

[He-Pin]

Let me merge this, I think this one should be ready to go, any improvement can come up after.