Support V4 signature

[clue]

Right now, this project implements the V2 signature which is supported by pretty much any S3 implementation. In the future, we should also support V4 signatures and reconsider the default signature version.

Link dump:

• https://docs.aws.amazon.com/en_pv/general/latest/gr/sigv4_changes.html
• https://github.com/tpyo/amazon-s3-php-class/blob/62ae202afaac4cc01feffe6ce54f1b7d36a8d79e/S3.php#L1995
• https://github.com/akeeba/s3/blob/4be75c333f61838d0eb58346e66d51977192a3ce/src/Signature/V4.php and others