Custom Branding: Relative URL should be allowed (#5572) (#5624)

When enabling the advanced setting `courier:ignoreFilterIfFieldNotInIndex`
Custom OpenSearch Query DSL filters could technically be applied to index
patterns that map to indices that are not exactly the same. Since the
custom query filter is a user input then users can really type anything
that they need. Or any field that they know is present but we do not know
for sure.

Therefore, we can check if the id which is the index pattern title to check
if we should apply the filter or not.

Issue resolved:
https://github.com/opensearch-project/dashboards-visualizations/issues/281

Issue could be closed:
#5423

---------







(cherry picked from commit d8cbc17)

Signed-off-by: Alankarsharma <alankar.sharma005@gmail.com>
Signed-off-by: Kawika Avilla <kavilla414@gmail.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: opensearch-trigger-bot[bot] <98922864+opensearch-trigger-bot[bot]@users.noreply.github.com>
Co-authored-by: Kawika Avilla <kavilla414@gmail.com>
Co-authored-by: Qingyang(Abby) Hu <abigailhu2000@gmail.com>

src/core/server/rendering/rendering_service.test.ts

@@ -195,6 +195,11 @@ describe('RenderingService', () => {
       const result = await service.isUrlValid('/', 'config');
       expect(result).toEqual(false);
     });
+
+    it('checks relative URL returns true', async () => {
+      const result = await service.isUrlValid('/demo/opensearch_mark_default.png', 'config');
+      expect(result).toEqual(true);
+    });
   });
 
   describe('isTitleValid()', () => {

src/core/server/rendering/rendering_service.tsx

@@ -372,6 +372,9 @@ export class RenderingService {
       this.logger.get('branding').error(`${configName} config is invalid. Using default branding.`);
       return false;
     }
+    if (url.startsWith('/')) {
+      return true;
+    }
     return await Axios.get(url, {
       httpsAgent: this.httpsAgent,
       adapter: AxiosHttpAdapter,