[5.6] SFTP driver

[mputkowski]

This PR adds SFTP storage driver, it allows to SSH key based authentication. SFTP driver requires league/flysystem-sftp so it should be added to composer file as in case of AWS or Rackspace drivers.

Example config:

'sftp' => [
    'driver' => 'sftp',
    'host' => 'example.com',
    'username' => 'username',
    'password' => 'password',
    'privateKey' => '/path/to/privateKey',
],

[deleugpn]

@sisve what's your take on composer changes on a point-release? You usually have some good crazy-case insight about bc changes.

[sisve]

I have been summoned! ;)

I see no issues with this. It's only a suggestion, and an opt-in feature. The potential for bc breaking changes is when you force a new dependency upon people, specially when such a dependency may be in conflict with existing code. This is not the case with this PR.

I guess my bitter self see the usual documentation problem in asking people that "yes, some versions of 5.6 supports SFTP, are you sure you are on the right one?". This is mostly a documentation issue where we need to be clear on what release contains this new feature.