Fix issue with KQL wildcard queries not properly escaping backslashes (#174464)

## Summary

Fixes https://github.com/elastic/kibana/issues/169709.

Prior to this PR, KQL queries against keyword fields would not properly
handle escaped special characters (such as backslash). This PR fixes the
behavior to properly re-escape special characters before sending them
along to the wildcard query.

### 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

### Release note

KQL queries against wildcards now properly handle escaped special
characters without requiring double-escaping.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

packages/kbn-es-query/src/kuery/functions/is.test.ts

@@ -253,6 +253,29 @@ describe('kuery functions', () => {
         expect(result).toEqual(expected);
       });
 
+      test('should create a wildcard query with backslashes properly escaped', () => {
+        const expected = {
+          bool: {
+            should: [
+              {
+                wildcard: {
+                  'machine.os.keyword': { value: '*\\\\*' },
+                },
+              },
+            ],
+            minimum_should_match: 1,
+          },
+        };
+        const node = nodeTypes.function.buildNode(
+          'is',
+          'machine.os.keyword',
+          '*\\\\*'
+        ) as KqlIsFunctionNode;
+        const result = is.toElasticsearchQuery(node, indexPattern);
+
+        expect(result).toEqual(expected);
+      });
+
       test('should support scripted fields', () => {
         const node = nodeTypes.function.buildNode(
           'is',

packages/kbn-es-query/src/kuery/functions/is.ts

@@ -159,7 +159,7 @@ export function toElasticsearchQuery(
         ? {
             wildcard: {
               [field.name]: {
-                value,
+                value: wildcard.toQueryStringQuery(valueArg),
                 ...(typeof config.caseInsensitive === 'boolean' && {
                   case_insensitive: config.caseInsensitive,
                 }),