Accept multipart checksums with part count (#19680)

Accept multipart uploads where the combined checksum provides the expected part count.

It seems this was added by AWS to make the API more consistent, even if the 
data is entirely superfluous on multiple levels.

Improves AWS S3 compatibility.

cmd/erasure-multipart.go

@@ -1231,7 +1231,7 @@ func (er erasureObjects) CompleteMultipartUpload(ctx context.Context, bucket str
 	}
 
 	if opts.WantChecksum != nil {
-		err := opts.WantChecksum.Matches(checksumCombined)
+		err := opts.WantChecksum.Matches(checksumCombined, len(parts))
 		if err != nil {
 			return oi, err
 		}