chore: Tweak withAttributes in Flow

[He-Pin]

Motivation:

To avoid #1525 , I checked all the Flow#withAttributes usage.

Modification:
Move the .withAttributes() inside the via(...)

Result:

1. consistent style
2. safer when using it with graphdsl.

[He-Pin]

@pjfanning I think we should have this in 1.2.0-M1 , it's some kind of bug fix.

[pjfanning]

Is there any way to add test coverage? Something like https://github.com/apache/pekko/blob/main/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowZipWithIndexSpec.scala#L60-L92 - which was added for #1525

[He-Pin]

Feel free to add it to m2, but actually the current fix not harm even without test.

Otherwise ,it will cause problem when using in graphdsl as the zipwithIndex was implemented.

[pjfanning]

I am uncomfortable with adding this in M1. There will be an M2 in a month or 2 and hopefully this can be added with some tests.