Create a planner rule to rewrite grouped mean calls

This depends on #18862.

Rewrite any call to `group(...) |> mean()` with something like the following:

```
ReadGroup(count, sum) |> map(fn: (r) => ({r with _value: float(v: r.sum) / float(v: r.count)}))
```

The `ReadGroup` call, which the output of is not designed yet, will return the aggregates together and they can be combined using a `map` call to find the mean of each group.

Start by reviewing dependent issue #18862 and the planner rule handling for grouped mean calls. Trace how `group(...) |> mean()` is represented, then verify the rewrite to `ReadGroup(count, sum) |> map(...)` against planner tests or existing rule patterns. Done means grouped mean expressions are rewritten with the intended aggregate inputs and per-group calculation.

Written by the indexing model from the issue text.