Support multiple OS package URLs

The host configuration format supports multiple provisioning URLs, see: https://git.glasklar.is/system-transparency/core/stboot/-/blob/d576dc81c6c6e0e49c689deb556fc791be2e5e5d/stboot.go#L620

However, stprov only supports populating that list with a single item (either using -r or a compiled-in template URL).