Files: SFTP support

[Dubzer]

Hi. It would be great to see SFTP support in Files UWP.
Currently, there is only three options to use SFTP in Windows:

1. WinSCP - an old app that is not very comfortable to use. (there is also MobaXterm, but it just works even slower)
2. Apps that create a virtual network drive. As they integrate with Windows Explorer, this is not a bad option, but path that Explorer uses is not Unix-style, so you can't just copy path to some file and paste it in terminal. (for ex. Z:\etc\nginx instead of /etc/nginx)
3. Total Commander and Far Manager SFTP plugins (which is not modern and more for old-school users)

Files UWP can take it advantages and get users who are looking for a modern SFTP client on Windows.
I've seen SFTP packages on nuget, so it might not be that difficult to implement

[issue-label-bot[bot]]

Issue-Label Bot is automatically applying the label feature_request to this issue, with a confidence of 0.92. Please mark this comment with :thumbsup: or :thumbsdown: to give our bot feedback!

Links: app homepage, dashboard and code for this bot.

[yaichenbaum]

Merging with #1342.