Allow setting SFTP/FTP username via Options or env var

[funkyshu]

Currently an SFTP and FTP pass the username as part of the URI Authority section:

fs := sftp.NewFileSysetm(
    sftp.WithOptions(
        &sftp.Options{
            Password: "my-secret",
        },
    ),
)
file, _ := fs.NewFile("[email protected]:22", "/some/path/file.txt")

// or 

_ := os.SetEnv("VFS_SFTP_PASSWORD", "my-secret")
file, _ := vfssimple.NewFile("sftp://[email protected]:22/some/path/file.txt")

However, there may be cases where you'd prefer to keep that auth separate. If we add Username as an Option field (set-able via env vars as well), a user-less authority should should work just as well:

fs := sftp.NewFileSysetm(
    sftp.WithOptions(
        &sftp.Options{
            Username: "bob",
            Password: "my-secret",
        },
    ),
)
file, _ := fs.NewFile("host.com:22", "/some/path/file.txt")

// or 

_ := os.SetEnv("VFS_SFTP_USERNAME", "bob")
_ := os.SetEnv("VFS_SFTP_PASSWORD", "my-secret")
file, _ := vfssimple.NewFile("sftp://host.com:22/some/path/file.txt")