s3: add support for --upload-header If-Match and If-None-Match

The If-Match and If-None-Match headers were being dropped rather
than implemented in the Put Object request to S3. These headers
make requests conditional which allow AWS S3 Bucket Policies to
prevent Object overwriting.

backend/s3/s3.go

@@ -4552,6 +4552,10 @@ func (o *Object) prepareUpload(ctx context.Context, src fs.ObjectInfo, options [
 			ui.req.ContentLanguage = aws.String(value)
 		case "content-type":
 			ui.req.ContentType = aws.String(value)
+		case "if-match":
+			ui.req.IfMatch = aws.String(value)
+		case "if-none-match":
+			ui.req.IfNoneMatch = aws.String(value)
 		case "x-amz-tagging":
 			ui.req.Tagging = aws.String(value)
 		default: