Use more explicit settings for S3 operations

It would be nice if the s3 connector didn't default to the `DefaultAWSCredentialsProviderChain` and instead took an implicit `S3Settings` for each operation. Similar to how standard library operations take an implicit `ExecutionContext`, users could simply import something like `akka.stream.alpakka.s3.S3Settings.implicit.default` if they want the default.

If not, this makes it easier to pass custom settings like `S3.download(bucket, bucketKey)(customSettings)` or define their own implicit (in my case, settings from `application.conf`.

Either way, an explicit action makes it more clear what's happening from reading the code without having to go to the documentation.

Start by tracing the S3 connector operations and their current credential and settings resolution. Compare the requested implicit S3Settings approach with the existing implicit ExecutionContext pattern; done means the supported operations make settings explicit or consistently use an implicit default, with behavior documented and covered by tests.

Written by the indexing model from the issue text.