AWS S3: Custom header injection

[vigoo]

Purpose

This PR adds the possibility to add custom headers to any of the AWS S3 operations.

References

The motivation was discussed in the following issue: #1938

Changes

• Adds a new factory class beside the existing S3 called S3WithHeaders that contains the same set of operations but modified with the capability to pass an S3Headers value to any of them.

Background Context

I assumed (based on the contributor guidelines) that binary compatibility must be maintained, so I created a new API S3WithHeaders beside the existing one. If this would not be true, the contents of this new object could be the new S3 instead.

[lightbend-cla-validator]

Hi @vigoo,

Thank you for your contribution! We really value the time you've taken to put this together.

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[vigoo]

CLA signed

[ennru]

There are some unused imports in your code
https://travis-ci.com/akka/alpakka/jobs/238787362#L973

and this needs extra parens for Scala 2.13
https://travis-ci.com/akka/alpakka/jobs/238787363#L1249

[ennru]

Please add the new methods to the existing S3. Binary-compatiblity is provided even when adding new methods, as long as the old signature is available and the class is not intended for extension.

[vigoo]

So I should rather add a xxxWithHeaders or something variant beside each existing method in S3?

[vigoo]

Ok so I realized that they can be just overloaded methods, I'm pushing the change soon

[ennru]

Looks much more regular now, great work.

I wonder if the S3Headers object should contain a val instance which to be returned where empty headers are required instead of creating a new every time.

[ennru]

This comment is obsolete now, right?

[vigoo]

I removed the obsolete comment and added an empty S3Headers value.

[ennru]

I'd suggest you do it like:

val empty = new S3Headers()
def apply() = empty
def create() = empty

[vigoo]

Would you prefer keeping the empty on the API too as I did or just hide it and use in apply and create?

[ennru]

LGTM.

[ennru]

Thank you, great work! We hope to see you around more.

[vigoo]

Thanks for merging!