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

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:

```yaml
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).