Add exists function to library

[digizeph]

Highlight

• add oneio::exists(path: &str) function to check if a local or remote file exists.
  • currently support local file, http(s) remote and s3 remote files checking

Example usages

    assert!(!oneio::exists("https://spaces.bgpkit.org/oneio/test_data_NOT_EXIST.json").unwrap());
    assert!(oneio::exists("https://spaces.bgpkit.org/oneio/test_data.json").unwrap());