[Feature Request] Enhance user-data used by autoinstall

[tai271828]

Description

In this pull request #8 we are starting to introduce autoinstall to complete the subiquity installation. So far the example only shows a very basic amount of configuration. We may add some more setup by the automatic subiquity installation. For example:

• install ssh server by default
• fetch public ssh keys if available

More ideas are welcome.

[dannf]

I like that we're helping users "bootstrap" an autoinstall configuration from ubuntu-server-netboot, but I'm wary about trying to document the autoinstall syntax/features too much here. I'd prefer if we could link out to official/general subiquity docs for that. Of course, if we want to extend our example to do just a few more common things, +1.