Allow additional base templates to be specified during `limactl create`

[jandubois]

It should be possible to run this command:

limactl create https://example.com/template.yaml ./secrets.yaml

And the effect should be the same as if the user has used this local template:

base: [https://example.com/template.yaml, ./secrets.yaml]

This allows the user to supply locally defined settings to an external template.

If secrets.yaml had been specified inside template.yaml, then it would have been resolved relative to the template location, and not relative to the current directory, i.e. it would have tried to fetch https://example.com/secrets.yaml.

For consistency the same should work for limactl start too (when it is used to create and start a new instance in a single step).