[ES|QL] Add to_lower and to_upper functions (elastic#175122)

## Summary

Syncing with https://github.com/elastic/elasticsearch/pull/104309/files


### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

packages/kbn-monaco/src/esql/lib/ast/definitions/functions.ts

@@ -137,6 +137,32 @@ export const evalFunctionsDefinitions: FunctionDefinition[] = [
       },
     ],
   },
+  {
+    name: 'to_lower',
+    description: i18n.translate('monaco.esql.definitions.toLowerDoc', {
+      defaultMessage: 'Returns a new string representing the input string converted to lower case.',
+    }),
+    signatures: [
+      {
+        params: [{ name: 'field', type: 'string' }],
+        returnType: 'string',
+        examples: ['from index | eval to_lower(field1)'],
+      },
+    ],
+  },
+  {
+    name: 'to_upper',
+    description: i18n.translate('monaco.esql.definitions.toUpperDoc', {
+      defaultMessage: 'Returns a new string representing the input string converted to upper case.',
+    }),
+    signatures: [
+      {
+        params: [{ name: 'field', type: 'string' }],
+        returnType: 'string',
+        examples: ['from index | eval to_upper(field1)'],
+      },
+    ],
+  },
   {
     name: 'trim',
     description: i18n.translate('monaco.esql.definitions.trimDoc', {