v5.0.12

What's Changed

• modify demo.c by @chen-honggang in #28
• add cos_check_bucket_exist cos_check_object_exist and object tagging api by @chen-honggang in #30
• check config params by @chen-honggang in #31
• opt presign demo by @chen-honggang in #32
• add config comment by @chen-honggang in #33
• add test_download_directory demo by @chen-honggang in #34
• adjust cos_demo.c for opt by @chen-honggang in #35

Full Changelog: v5.0.11...v5.0.12