Enhancement: floppy_command section similar to boot_command

_This issue was originally opened by @mcandre as hashicorp/packer#8057. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Some classic operating systems such as OS/2 Warp require floppy disks to be inserted prior to loading ISO's. And so it would be helpful for Packer to provide a `floppy_command` section, similar to `boot_command`, where users can insert and remove virtual floppy diskettes with custom delays between data loads. The user configuration would look like:

```json
floppy_command: [
"",
"",
"",
"",
"",
"",
""
"",
"",
"",
""
],
iso_url: "winworldpc.com/os2/...",
boot_command: [
"..."
]
```

As a workaround, I found a minor abuse of the `guestproperty wait` subcommand for the VirtualBox hypervisor, e.g. `guestproperty wait goodnight-moon --timeout `, followed by `controlvm 'os/2' keyboardputscancode`... for the Enter scancodes. Followed by `controlvm savestate` to free the VM lock for the rest of the Packer build.

This happens to not depend on VirtualBox guest drivers, and `guestproperty wait` even exits with a zero code for now. But this could easily break in the future, and digging around with hypervisor details kind of defeats the purpose of using Packer in the first place.