add miniox example

example/miniox_example.dart

+import 'dart:io';
+
+import 'package:minio/io.dart';
+import 'package:minio/minio.dart';
+
+void main() async {
+  final minio = Minio(
+    endPoint: 'play.min.io',
+    accessKey: 'Q3AM3UQ867SPQQA43P2F',
+    secretKey: 'zuf+tfteSlswRu7BJ86wekitnifILbZam1KYY3TG',
+  );
+
+  await minio.fPutObject('testbucket', 'test.png', 'example/custed.png');
+
+  final stat = await minio.statObject('testbucket', 'test.png');
+  assert(stat.size == File('example/custed.png').lengthSync());
+}
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