Make `iso_file` more configurable

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#### Description
When creating a VM, I can specify `iso_file` but no other configuration e.g. `device`. I can, however, specify this if I use the `additional_iso_files` array. The issue is that `iso_file` is mandatory, but does not let me configure it correctly.

#### Use Case(s)
My specific setup is running Proxmox on a Raspberry Pi. The issue I have is that the ISO has to be a `scsi` device, but the required `iso_file` attaches it as `ide`.

#### Potential configuration
I can see three potential ways that this could be fixed (this is not exhaustive, I'm sure there are other solutions):
1. Make `iso_file`* optional
- Would avoid breaking existing configurations that people have
2. Make `iso_file`* optional with `additional_iso_files` i.e. require one of them
- I think this would be the best option, as it won't break people's current configurations
3. Remove `iso_file`* and use `additional_iso_files`
- I think this would be the ideal solution as I don't really understand why the first ISO is treated special instead of just part of the list of ISOs, but would probably break a lot of exisiting configurations
- It would also probably make sense for `additional_iso_files` to be renamed to `iso_files` in this case

\* When I say `iso_file` I am also referring to the other fields that accompany it e.g. `iso_url`, `iso_storage_pool`, `iso_checksum`

#### Potential References
Documentation for [`iso_file`](https://www.packer.io/plugins/builders/proxmox/iso#iso_file)
Documentation for [`additional_iso_files`](https://www.packer.io/plugins/builders/proxmox/iso#additional_iso_files)