Use SSH Agent

My setup is like this: my (encrypted) private key is stored in `~/.ssh/id_rsa`. The passphrase for this key is stored in Keychain. The agent is launched automatically by the system, the key is inserted automatically. [[Docs](https://developer.apple.com/library/content/technotes/tn2449/_index.html)] It would be great if Postico would just use the agent instead of trying to load the encrypted key from disk.

Benefits:

- Security: Only the OS (access group com.apple.ssh.passphrases) can access the passphrase of the private key.
- Ease of use: Other tools (e.g. ssh, rsync, git, Cyberduck, Sequel Pro, [basically any other tool that I use], and soon Postico?) "just work" because they use the agent, no need to set path to private key or anything.

[libssh2 already supports this](https://www.libssh2.org/examples/ssh2_agent.html) so it shouldn't be hard to implement.

Start with the Apple TN2449 documentation and the libssh2 SSH agent example linked in the issue, then locate Postico's existing encrypted-key loading path. Done means Postico can use the system SSH agent without loading the encrypted private key from disk, while preserving the existing connection flow.

Written by the indexing model from the issue text.