Include an optional initial query in the ruru config that sets the editor contents regardless of user history.
### Feature description

Currently, the ruru config supports a `defaultQuery` option that sets the editor contents if the user does not have any stored history. It would be helpful to have a similar paramter that is respected regardless of stored user history.

### Motivating example

<!-- What motivated this feature request? Why is this feature useful to other
      people? What other use cases can you think of for it?  -->
This feature would allow things like query sharing via links with url encoded queries. e.g. 

```typescript
const plugin: GraphileConfig.Plugin = {
  name: "RuruQueryParamsPlugin",
  grafserv: {
    hooks: {
      ruruHTMLParts(_info, parts, extra) {
        parts.headerScripts += `
<script>
RURU_CONFIG.query = "{ testing: __typename }";
</script>
`;
      }
    }
  }
}
```

### Supporting development

I [tick all that apply]:

- [ ] am interested in building this feature myself
- [ ] am interested in collaborating on building this feature
- [x] am willing to help testing this feature before it's released
- [ ] am willing to write a test-driven test suite for this feature (before it exists)
- [x] am a [Graphile sponsor](https://www.graphile.org/sponsor/) ❤️
- [x] have an active [support or consultancy contract](https://www.graphile.org/support/) with Graphile
