MV_MEDIAN

Converts a multivalued field into a single valued field containing the median value. For example:
```esql
ROW a=[3, 5, 1]
| EVAL median_a = MV_MEDIAN(a)
```

It can be used by any numeric field type and returns a value of the same type. If the
row has an even number of values for a column the result will be the average of the
middle two entries. If the field is not floating point then the average rounds down:
```esql
ROW a=[3, 7, 1, 6]
| EVAL median_a = MV_MEDIAN(a)
```
