Make it clear that "SSH Private Key" falls back to SSH agent

[minfrin]

Describe the bug

When configuring mountain duck to connect to an SSH server using an SSH private key supplied by an ssh-agent, you are forced to specify "None".

This gives end user impression that ssh-agent is not supported.

To Reproduce
Steps to reproduce the behavior:

1. Configure SFTP server
2. Make sure ssh-agent is running and works.
3. Select "SSH Private Key: None"
4. See ssh-agent being used.

Expected behavior

Instead of "None", the option should clearly indicate the difference between no key, and an ssh-agent.

Screenshots

Desktop (please complete the following information):

• OS: MacOS
• Version 4.13.4 (20921)

Log Files

Additional context