External driver, for cloud and physical

### Description

Add a generic driver for external virtual machines and physical servers.

It is similar to running `ssh`, but includes all the Lima features like agent.

---

The user is supposed to have an existing server, with key-based ssh login.

Previously it was hardcoding `127.0.0.1` as the hostname

Host keys are being used (and should be set up), to verify the external server.

Previously it was hardcoding `StrictHostKeyChecking=no`

A `guest-install` command is added, to install lima-guestagent and nerdctl-full.

The external machine lifecycle (boot/reboot/etc) is managed outside of Lima.

----

* #2000
* #2028

Tested with Digital Ocean, and with Raspberry Pi.

Note: this driver does **not** create cloud instances.

* https://github.com/lima-vm/lima/discussions/2029
* [docker machine cloud drivers](https://github.com/machine-drivers/docker.github.io/blob/173d3c65f8e7df2a8c0323594419c18086fc3a30/machine/get-started-cloud.md) (archived 2019)