[discuss] Sorting of stream configs

Today when creating an integration configs, the streams configs inside an input are sorted alphabetical. Unfortunately in the case of the system package, it means on top configs show up like core or entropy which are not used that often. Instead `memory` and others should be at the top.

To solve this issue I'm proposing we add support for an optional "priority/order" config option in the [stream definition](https://github.com/elastic/package-registry/blob/master/dev/packages/example/reference/1.0.0/dataset/reference/manifest.yml#L20).

This would then look similar to:

```
streams:
- input: logs
priority: 1
```

Kibana would then sort first based on priority and second alphabetical. The ones without a priority would go at the end.