AWS S3: Add S3.listBuckets

[mdedetrich]

This PR adds the AWS S3 list buckets call (see https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListBuckets.html). The call itself is trivial however there are some non conventional differences

• This seems to be one of the few S3 calls that returns a list of entities that is not paginated (i.e. the list of entities is a XML node list in the response). Due to this its an open question whether the S3.listBuckets method should just return a Future[Seq[ListBucketsResultContents]] or the current version which is a Source[ListBucketsResultContents, NotUsed]. I opted for the Source variant due to consistency with the rest of the S3 API (every single other method in S3 that returns a list of entities is a Source) plus there is a chance that AWS may add a paginated list-buckets call in the future in which case its trivially easy to use the newly updated version. I don't feel strongly about it either way however, so if you want me to change it to Future[Seq[ListBucketsResultContents]] then just let me know
• The base construction of the URI is also very different to any other S3 call, hence why the listBuckets call in HttpRequests.scala is manually constructing the URI itself, i.e. there isn't any Path/Virtual access style and the authority is hardcoded to s3.amazonaws.com (for AWS) since the call is region agnostic. I have tested this both with Minio and a real AWS S3 account so I can confirm its working

[mdedetrich]

Test failure for CI / ScalaDoc, Documentation with Paradox (pull_request) is unrelated (link validator is failing).

[ennru]

LGTM. Great work, as always.