ESQL: Expressions in aggregate functions

### Description

It would be great to have support for expressions like
```
from employees | stats avg(languages + 1) by whatever_field
```
or simply
```
from employees | stats count(1) by whatever_field
```

Currently, this can be worked around using `eval`, e.g.:
```
from employees | eval langsplusone = languages+1 | stats avg(langsplusone) by whatever_field
```