Add SearchStrategyRegistry and defaultSearchStrategy to support existing search behavior, and integrate it with CallClient.

src/ui/public/courier/fetch/__tests__/call_client.js

@@ -273,9 +273,7 @@ describe('callClient', () => {
   describe('searchRequests with multiple searchStrategies map correctly to their responses', () => {
     const search = ({ searchRequests }) => {
       return {
-        searching: Promise.resolve({
-          responses: searchRequests.map(searchRequest => searchRequest.__testId__),
-        }),
+        searching: Promise.resolve(searchRequests.map(searchRequest => searchRequest.__testId__)),
         failedSearchRequests: [],
         abort: () => {},
       };

src/ui/public/courier/search_strategy/default_search_strategy.js

@@ -68,12 +68,12 @@ export const defaultSearchStrategy = {
       failedSearchRequests,
     } = await serializeAllFetchParams(allFetchParams, searchRequests, serializeFetchParams);
 
-    const searching = es.msearch({ body: serializedFetchParams }).then(({ responses }) => responses);
-    const abort = searching.abort;
+    const searching = es.msearch({ body: serializedFetchParams });
 
     return {
-      searching,
-      abort,
+      // Unwrap the responses object returned by the es client.
+      searching: searching.then(({ responses }) => responses),
+      abort: searching.abort,
       failedSearchRequests,
     };
   },