Fix ProxyJump to support SSH ports and host aliases

[andrewleech]

ET currently fails to connect through jump hosts when the jump host uses a non-standard SSH port or when ProxyJump references an SSH config Host alias.

For example, with this SSH config:

Host jumphost
  HostName jump.example.com
  Port 2222

Host destination
  HostName dest.example.com
  ProxyJump jumphost

Running et destination fails with Could not reach the ET server: jumphost:2022

The issue is that the ProxyJump parsing code was stripping SSH ports from the jump host value:

if (colonIndex != string::npos) {
  jumphost = proxyjump.substr(0, colonIndex);  // Strips :2222
}

Additionally, Host aliases like "jumphost" weren't being resolved to actual hostnames, so ET tried to connect to the alias name instead of the configured hostname.

This PR fixes both issues by:

• Preserving the full ProxyJump value [user@]host[:port] when passing to SSH via -J flag
• Extracting just the hostname from the ProxyJump value for ET socket connections
• Looking up Host aliases in SSH config to get the actual hostname

Tested with the config above. Before the fix et destination fails, after it successfully connects via jump.example.com:2222 (SSH) -> destination (ET).

No breaking changes expected - existing ProxyJump configurations should continue to work, though I don't currently have a working jumphost setup to verify backwards compatibility directly.