Support private key for SFTP/SSH storage connection

[alexram1313]

Important

Please follow this template!

---

What's happened?

Our service's storage server supports SFTP/SSH but requires private key authentication. phpFox 4.8.3 does not include a field to specify a path to a private key.

Since 4.8.0, our storage server has been incompatible with phpFox's storage solution, while it had worked using the phpFox CDN interface.

Steps to reproduce:

1. Navigate to /admincp/setting/storage/sftp/
2. Create/Edit an instance of an SFTP server
3. No private key field is present.

What's expected?

A private key field should be present.

Browsers and Devices tested

(Example: Chrome on iPhone X, Safari on Macbook, Miscrosoft Edge on Windown 10, Firefox on Ubuntu 16.04, ...)

Chrome, Firefox, macOS 11.0.1, Android 11

Server information

(Example: CentOS 7, php 7.1 apache)

nginx PHP 7.0 FPM, Ubuntu 16.04.7 LTS

phpFox version

(Example: phpFox 4.6.0)

4.8.3

Screenshots