s3: add initial --s3-directory-bucket to support AWS Directory Buckets

This will ensure no Content-Md5 headers are sent and ensure ETags are not
interpreted as MD5 sums. X-Amz-Meta-Md5chksum will be set on all objects
whether single or multipart uploaded.

This also sets "no_check_bucket = true".

This is enough to make the integration tests pass, but there are some
limitations as noted in the docs.

See: https://forum.rclone.org/t/support-s3-directory-bucket/47653/

docs/content/s3.md

@@ -2294,6 +2294,21 @@ You can also do this entirely on the command line
 
 This is the provider used as main example and described in the [configuration](#configuration) section above.
 
+### AWS Directory Buckets
+
+From rclone v1.69 [Directory Buckets](https://docs.aws.amazon.com/AmazonS3/latest/userguide/directory-buckets-overview.html)
+are supported.
+
+You will need to set the `directory_buckets = true` config parameter
+or use `--s3-directory-buckets`.
+
+Note that rclone cannot yet:
+
+- Create directory buckets
+- List directory buckets
+
+See [the --s3-directory-buckets flag](#s3-directory-buckets) for more info
+
 ### AWS Snowball Edge
 
 [AWS Snowball](https://aws.amazon.com/snowball/) is a hardware