Group by custom eval field/tag and a list

Hello!

I'm building a task template and trying to group by a custom eval field/tag as well as whatever group is passed into the template. For example, my yaml file would have a group like this:
```
...
group:
type: list
value:
- type: string
value: 'category_1'
...
```

and I would want to be able to group like so:

```
...
|groupBy('compVal',group)
...
```

Compval is a value that is being made using eval.

I understand that I could manually configure it within the yaml file, but it would be a pain for everyone using this template to remember to do that.

Is there any way to accomplish this? I have not seen any documentation on combining lists.

Start by tracing the groupBy('compVal', group) template expression and how custom eval fields and list-valued groups are represented in Kapacitor. Determine whether combining the evaluated value with the template's group requires a documented capability or broader grouping design; done means the requested grouping works for the shown YAML and expression, with coverage for list combinations.

Written by the indexing model from the issue text.