[keep-web] [s3] Implement NextContinuationToken per ListObjectsV2 API
According to https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListObjectsV2.html the ListObjectsV2 API should return NextContinuationToken when isTruncated is true, and accept that value as the continuation-token parameter in a subsequent request.
Currently, keep-web only implements ListObjectsV1 paging (i.e., it sends NextMarker and accepts that value as a marker parameter).
To fix this we should look for the list-type=2 parameter, and if it's set, send NextContinuationToken and accept continuation-token, instead of sending NextMarker and accepting marker.
Probably best to use a base64-encoded filename as a token, rather than the plain filename, in case client implementations rely on the assumption (which appears to be true with AWS) that the token won't have special characters.