@kbn/openapi-generator: Support object keys which need to be surround…

…ed by quotes (elastic#172049)

## Summary

❓ Are there tests for @kbn/openapi-generator? I couldn't find them ❓ 

Our team had a `@timestamp` field in an API response, this caused the
zod schema to break because the key was not being escaped in the object
e.g:

```js
export const AssetCriticalityRecord =       CreateAssetCriticalityRecord.and(z.object({
      // ❌ invalid JS
      @timestamp: z.string().datetime(),
  }));
```

With the fix, the `@timestamp` key is surrounded by quotes.

```js
export const AssetCriticalityRecord =       CreateAssetCriticalityRecord.and(z.object({
     // ✅ 
      '@timestamp': z.string().datetime(),
  }));
```

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

packages/kbn-openapi-generator/src/template_service/register_helpers.ts

@@ -47,4 +47,7 @@ export function registerHelpers(handlebarsInstance: typeof Handlebars) {
   handlebarsInstance.registerHelper('isUnknown', (val: object) => {
     return !('type' in val || '$ref' in val || 'anyOf' in val || 'oneOf' in val || 'allOf' in val);
   });
+  handlebarsInstance.registerHelper('startsWithSpecialChar', (val: string) => {
+    return /^[^a-zA-Z0-9]/.test(val);
+  });
 }

packages/kbn-openapi-generator/src/template_service/templates/zod_schema_item.handlebars

@@ -80,7 +80,11 @@ z.unknown()
       * {{{description}}} 
       */
       {{/if}}
-      {{@key}}: {{> zod_schema_item requiredBool=(includes ../required @key)}},
+      {{#if (startsWithSpecialChar @key)}}
+        '{{@key}}': {{> zod_schema_item requiredBool=(includes ../required @key)}},
+      {{else}}
+        {{@key}}: {{> zod_schema_item requiredBool=(includes ../required @key)}},
+      {{/if}}
     {{/each}}
   })
   {{~#if (eq additionalProperties false)}}.strict(){{/if~}}