Adding `MV_APPEND(value1, value2)` function, that appends two values
creating a single multi-value. If one or both the inputs are
multi-values, the result is the concatenation of all the values, eg.
```
MV_APPEND([a, b], [c, d]) -> [a, b, c, d]
```
~I think for this specific case it makes sense to consider `null` values
as empty arrays, so that~ ~MV_APPEND(value, null) -> value~ ~It is
pretty uncommon for ESQL (all the other functions, apart from
`COALESCE`, short-circuit to `null` when one of the values is null), so
let's discuss this behavior.~
[EDIT] considering the feedback from Andrei, I changed this logic and
made it consistent with the other functions: now if one of the
parameters is null, the function returns null
