ESQL: Make Categorize usable in aggs when identical to a grouping (#117835) (#117895)

Cases like `STATS MV_APPEND(cat, CATEGORIZE(x)) BY cat=CATEGORIZE(x)` should work, as they're moved to an EVAL by a rule.

Also, these cases were discarded, as they fail because of other verifications (Which also fail for BUCKET):
```
STATS x = category BY category=CATEGORIZE(message)
STATS x = CATEGORIZE(message) BY CATEGORIZE(message)
STATS x = CATEGORIZE(message) BY category=CATEGORIZE(message)

Author: ivancea

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

@@ -503,6 +503,27 @@ FROM employees
 //end::reuseGroupingFunctionWithExpression-result[]
 ;
 
+reuseGroupingFunctionImplicitAliasWithExpression#[skip:-8.13.99, reason:BUCKET renamed in 8.14]
+FROM employees
+| STATS s1 = `BUCKET(salary / 100 + 99, 50.)` + 1, s2 = BUCKET(salary / 1000 + 999, 50.) + 2 BY BUCKET(salary / 100 + 99, 50.), b2 = BUCKET(salary / 1000 + 999, 50.)
+| SORT `BUCKET(salary / 100 + 99, 50.)`, b2
+| KEEP s1, `BUCKET(salary / 100 + 99, 50.)`, s2, b2
+;
+
+ s1:double | BUCKET(salary / 100 + 99, 50.):double | s2:double   | b2:double
+351.0      |350.0      |1002.0       |1000.0
+401.0      |400.0      |1002.0       |1000.0
+451.0      |450.0      |1002.0       |1000.0
+501.0      |500.0      |1002.0       |1000.0
+551.0      |550.0      |1002.0       |1000.0
+601.0      |600.0      |1002.0       |1000.0
+601.0      |600.0      |1052.0       |1050.0
+651.0      |650.0      |1052.0       |1050.0
+701.0      |700.0      |1052.0       |1050.0
+751.0      |750.0      |1052.0       |1050.0
+801.0      |800.0      |1052.0       |1050.0
+;
+
 reuseGroupingFunctionWithinAggs#[skip:-8.13.99, reason:BUCKET renamed in 8.14]
 FROM employees
 | STATS sum = 1 + MAX(1 + BUCKET(salary, 1000.)) BY BUCKET(salary, 1000.) + 1