ESQL: Optimize aggregations over literals/constant expressions

### Description

Aggregations over literals and constant expressions can often be optimized so that no aggregation is evaluated.

Example:
```
from employees | stats m = max(1)
```
is equivalent to
```
from employees | eval m = 1 | keep m
```

Perform this optimization where possible (`count(1)` being a case where it's not possible).