Provide analyticsApi implementation to forward to multiple services

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

.changeset/veka-fingrar-drar.md

@@ -0,0 +1,28 @@
+---
+'@backstage/core-app-api': patch
+---
+
+If you'd like to send analytics events to multiple implementations, you may now
+do so using the `MultipleAnalyticsApi` implementation provided by this package.
+
+```tsx
+import { MultipleAnalyticsApi } from '@backstage/core-app-api';
+import {
+  analyticsApiRef,
+  configApiRef,
+  storageApiRef,
+  identityApiRef,
+} from '@internal/backstage/core-plugin-api';
+import { CustomAnalyticsApi } from '@internal/analytics';
+import { VendorAnalyticsApi } from '@vendor/analytics';
+
+createApiFactory({
+  api: analyticsApiRef,
+  deps: { configApi: configApiRef, identityApi: identityApiRef, storageApi: storageApiRef },
+  factory: ({ configApi, identityApi, storageApi }) =>
+    MultipleAnalyticsApi.withApis([
+      VendorAnalyticsApi.fromConfig(configApi, { identityApi }),
+      CustomAnalyticsApi.fromConfig(configApi, { identityApi, storageApi }),
+    ]),
+}),
+```