[S3] Add support for Trailing checksum headers

[Slamdunk]

Recently AWS S3 introduced the possibility to send content-length and hash at the end of the HTTP request:
https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity.html#trailing-checksums

This opens up the possibility to send a stream without having the whole content first, which is expecially useful where you don't have enough space in the disk or memory to handle a big file before sending it (i.e. dumping a database directly into S3).