Providing a set of values to a single feature `option`

This proposal is an enhancement idea to the current [dev container features proposal](https://github.com/devcontainers/spec/blob/main/proposals/devcontainer-features.md).

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Breaking off from https://github.com/devcontainers/spec/issues/44#issuecomment-1167687113 to address @Chuxel 's option (1).

There are several ways we could pass an array _of options_ to an individual features.

```jsonc
features: {
apt: {
"packages": ["vim", "jq", "curl"] // option A
"packages": "vim jq curl" // option B
}
}
```

Under the hook, this would be converted to an environment variable for the underlying shell script to handle.

`_BUILD_ARG_APT_GET_PACKAGES="vim jq curl"`