RFD 74: Add SFTP Support

[capnspacehook]

Rendered file here: https://github.com/gravitational/teleport/blob/capnspacehook/rfd/0074-sftp-support/rfd/0074-sftp-support.md

[capnspacehook]

@capnspacehook Haven't looked in detail yet, but one comment from the product team is that we should have an option in the RBAC to disable SFTP. Maybe an option in the role? I don't remember if we have a similar knob for scp already. Can you please make sure the RFD covers this?

@r0mant I can add that to the RFD sure. I don't see a knob for controlling scp in the roles reference, so should we add one for both scp and sftp?

[capnspacehook]

I made some changes to how tsh will be changed after talking with @r0mant, I'd appreciate another review

[capnspacehook]

Also just thought of this, should the SCP event be modified to add a field specifying what protocol was used? And should an event be added when a file copy is denied via the disable_file_copy role option?

[r0mant]

Also just thought of this, should the SCP event be modified to add a field specifying what protocol was used? And should an event be added when a file copy is denied via the disable_file_copy role option?

@capnspacehook I think both are good suggestions.

[zmb3]

This one looks good to go, can we get it merged @capnspacehook?