Allow to download files in ovf/ova builder

_This issue was originally opened by @azr as hashicorp/packer#7609. 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._

Currently, the virtualbox-ovf builder can only use local files.

I think we could allow to download files for the ovf/ova builder.

* For ova files we could allow downloads simply.
* For ovf files & using the go-getter we could allow archives downloads, we could reference the ovf file in there ? By for example forcing users to name the files in the archive in a specific manner. No sure if files in there are using a relative path & etc ?

Previous issues: https://github.com/hashicorp/packer/issues/1195 https://github.com/hashicorp/packer/pull/3982