Add global `--ssh-args` argument

[schlessera]

Some SSH functionality can currently not be used (or only through workarounds) because we cannot directly pass arguments to SSH, we only have the connection string to communicate with it.

One of these limitations can be seen here: https://github.com/wp-cli/wp-cli/issues/5100

Adding a new global argument to pass arguments to SSH would make this more flexible and open up access to the full functionality of SSH.

As we are using the --ssh flag for access to docker & vagrant as well, this would also allow the user to pass arguments to these as well.

[nickbasham]

This would be a great addition, just wanted to add my vote!