Runtime Config querySource listener (#879)

Listen to runtime config updates for 'querySource' and set the corresponding method in ANSWERS.
 
The theme must be pointed to an SDK version which includes this change for this to work: yext/answers-search-ui#1464.

J=SLAP-1438
TEST=manual

Test setting the query source in iframe and non-iframe experiences with runtime config and observe the correct source being sent to the API

static/js/answers-experience.js

@@ -1,6 +1,7 @@
 import DeferredPromise from './deferred-promise';
 import analyticsListener from './runtime-config-listeners/analytics';
 import sessionTrackingListener from './runtime-config-listeners/session-tracking';
+import querySourceListener from './runtime-config-listeners/query-source';
 
 /**
  * @typedef {import('./runtime-config.js').RuntimeConfigListener} RuntimeConfigListener
@@ -12,7 +13,8 @@ export default class AnswersExperience {
     this.AnswersInitializedPromise = new DeferredPromise();
     this._runtimeConfigListeners = [
       analyticsListener,
-      sessionTrackingListener
+      sessionTrackingListener,
+      querySourceListener
     ];
 
     this._registerRuntimeConfigListeners();

static/js/runtime-config-listeners/query-source.js

@@ -0,0 +1,9 @@
+/**
+ * @type {import('../runtime-config.js').RuntimeConfigListener}
+ */
+ export default {
+  key: 'querySource',
+  callback: value => {
+    ANSWERS.setQuerySource(value);
+  }
+}