ES|QL: add MV_APPEND function (#107001)

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

x-pack/plugin/esql-core/src/test/resources/mapping-multi-field-variation.json

@@ -8,6 +8,8 @@
         "keyword" : { "type" : "keyword" },
         "date" :  { "type" : "date" },
         "date_nanos": { "type" : "date_nanos" },
+        "long" :  { "type" : "long" },
+        "ip" :  { "type" : "ip" },
         "unsupported" : { "type" : "ip_range" },
         "some" : {
             "properties" : {

x-pack/plugin/esql/qa/testFixtures/src/main/resources/date.csv-spec

@@ -1076,6 +1076,23 @@ required_capability: agg_values
 [1955-01-21T00:00:00Z, 1957-05-23T00:00:00Z, 1959-12-03T00:00:00Z] | null
 ;
 
+
+mvAppendDates
+required_capability: fn_mv_append
+
+FROM employees
+| WHERE emp_no == 10039 OR emp_no == 10040
+| SORT emp_no
+| EVAL dates = mv_append(birth_date, hire_date)
+| KEEP emp_no, birth_date, hire_date, dates
+;
+
+emp_no:integer | birth_date:date      | hire_date:date       | dates:date
+10039          | 1959-10-01T00:00:00Z | 1988-01-19T00:00:00Z | [1959-10-01T00:00:00Z, 1988-01-19T00:00:00Z]
+10040          | null                 | 1993-02-14T00:00:00Z | null 
+;
+
+
 implicitCastingNotEqual
 required_capability: string_literal_auto_casting
 from employees | where birth_date != "1957-05-23T00:00:00Z" | keep emp_no, birth_date | sort emp_no | limit 3;

x-pack/plugin/esql/qa/testFixtures/src/main/resources/floats.csv-spec

@@ -523,6 +523,25 @@ required_capability: agg_values
 [1.7, 1.83, 2.05] | null
 ;
 
+
+mvAppend
+required_capability: fn_mv_append
+
+FROM employees
+| WHERE emp_no == 10008 OR emp_no == 10021
+| EVAL d = mv_append(salary_change, salary_change),
+       i = mv_append(salary_change.int, salary_change.int),
+       i2 = mv_append(emp_no, salary_change.int),
+       i3 = mv_append(emp_no, emp_no),  
+       s = mv_append(salary_change.keyword, salary_change.keyword)
+| KEEP emp_no, salary_change, d, i, i2, i3, s;
+
+emp_no:integer | salary_change:double    | d:double                                       | i:integer              | i2:integer         | i3:integer     | s:keyword
+10008          | [-2.92,0.75,3.54,12.68] | [-2.92,0.75,3.54,12.68,-2.92,0.75,3.54,12.68]  | [-2,0,3,12,-2,0,3,12]  | [10008,-2,0,3,12]  | [10008, 10008] | [-2.92,0.75,12.68,3.54,-2.92,0.75,12.68,3.54]
+10021          | null                    | null                                           | null                   | null               | [10021, 10021] | null
+;
+
+
 signumOfPositiveDouble#[skip:-8.13.99,reason:new scalar function added in 8.14]
 row d = to_double(100) | eval s = signum(d);