Facilitate creation of ESMValCore Dataset objects from datastores

**Is your feature request related to a problem? Please describe.**

See https://github.com/ESMValGroup/ESMValCore/pull/2690.

We're working on adding intake-esm support to ESMValCore/ESMValTool. I think one thing that would make this a lot more straightforward would be to add the ability to output an ESMValCore `Dataset` object from an `esm_datastore`. I'm envisaging something like

```python
esm_ds = intake.open_esm_datastore(...)
esm_ds = esm_ds.search(...)

dataset = esm_ds.to_dataset(...)
```

**Describe the solution you'd like**

I think it would be wise to go one of two ways here:
1. Make ESMValCore an optional dependency, in order to avoid making the dependencies of intake-esm too numerous.
2. Double down on the intake plugin architecture and add an entrypoint that allows users to easily add extra methods to esm datastores - similar to the xarrays `register_dataset_accessor` system. Then we can stick the intake-esm / ESMValCore bridging code in a separate repo.

**Additional context**
https://github.com/ESMValGroup/ESMValCore/pull/2690, https://github.com/ACCESS-NRI/ESMValTool-workflow/issues/201#issuecomment-2846520663, https://github.com/ACCESS-NRI/ESMValTool-workflow/issues/202#issuecomment-2846514853

This would also probably require some naming refinements, because `to_dataset` would get confusing with `to_dataset_dict` being completely unrelated. Maybe `to_iris` would be a better name.