Add new capability for function SPACE.

Author: chrisberkhout

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

@@ -1596,6 +1596,7 @@ emp_no:integer | languages:integer | first_name:keyword
 ;
 
 space
+required_capability: space
 // tag::space[]
 ROW message = CONCAT("Hello", SPACE(1), "World!");
 // end::space[]
@@ -1607,13 +1608,15 @@ Hello World!
 ;
 
 spaceLength
+required_capability: space
 ROW len = LENGTH(SPACE(12));
 
 len:integer
 12
 ;
 
 spaceCalculated
+required_capability: space
 ROW inner_width = 20, title = "Title"
 | EVAL lspace = SPACE((inner_width-LENGTH(title))/2),
        rspace = SPACE(inner_width-LENGTH(lspace)-LENGTH(title)),
@@ -1625,20 +1628,23 @@ inner_width:integer | centered_title:keyword
 ;
 
 spaceZero
+required_capability: space
 ROW s = SPACE(0);
 
 s:keyword
 ""
 ;
 
 spaceNull
+required_capability: space
 ROW s = SPACE(null);
 
 s:keyword
 null
 ;
 
 spaceNegative
+required_capability: space
 FROM employees | SORT emp_no | LIMIT 1 | EVAL s = SPACE(-LENGTH(first_name)) | KEEP s;
 
 warning:Line 1:51: evaluation of [SPACE(-LENGTH(first_name))] failed, treating result as null. Only first 20 failures recorded.

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -269,7 +269,12 @@ public enum Cap {
         /**
          * Allow mixed numeric types in coalesce
          */
-        MIXED_NUMERIC_TYPES_IN_COALESCE;
+        MIXED_NUMERIC_TYPES_IN_COALESCE,
+
+        /**
+         * Support for requesting the "SPACE" function.
+         */
+        SPACE;
 
         private final boolean snapshotOnly;
         private final FeatureFlag featureFlag;