Add options for custom exe paths and extension messages

[Hill-98]

If appendWindowsPath = false, then wsl2-ssh-agent will fail to start, you can add options to allow custom ·ssh.exe· and powershell.exe paths.

If the Windows ssh-agent is manually upgraded to OpenSSH 8.9+, or a different SSH Agent implementation is used, extended messages are available and we can add options to control it.

[mame]

I think it's good to have. Is the following strategy good?

• If a user explicitly gives a command-line option (e.g., -ssh-command /path/to/ssh.exe or -powershell-command /path/to/powershell.exe), use it.
• If not given, look for ssh.exe in the PATH. If not found, check if /mnt/c/Windows/System32/OpenSSH/ssh.exe and /mnt/c/Windows/System32/WindowsPowerShell/v1.0/PowerShell.exe" are available.

It would be great if you could send me a PR. Or, I'll probably do it myself a little later.