Add ability to set file permissions after writing an SFTP file

[funkyshu]

Is your feature request related to a problem? Please describe.
Some servers can't/won't set umasks properly and have requested changes to a file permission after upload.

Describe the solution you'd like
Add an SFTP-specific Option that allows you to set default permissions for newly created/updated files on open for write or touch.

Describe alternatives you've considered
Adding Chmod(string) error to the File interface. However this means cloud fs's like S3/GCS/Azure would have end up with no-op functions. For not make this SFTP-specific with a backend-specific option.