Address PR review comments

- Make ExtensionPointFactoryMiddleware an opaque type via new
  @internal/backend package using OpaqueType from @internal/opaque
- Use options object for createExtensionPointFactoryMiddleware
- Make middleware function async (returns Promise<T>)
- Remove extensionPointFactoryMiddleware from createBackend, keep only
  on createSpecializedBackend
- Export defaultServiceFactories from @backstage/backend-defaults

Signed-off-by: Jack Palmer <jackpalmer@spotify.com>

yarn.lock

@@ -9977,6 +9977,14 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@internal/backend@workspace:packages/backend-internal":
+  version: 0.0.0-use.local
+  resolution: "@internal/backend@workspace:packages/backend-internal"
+  dependencies:
+    "@backstage/cli": "workspace:^"
+  languageName: unknown
+  linkType: soft
+
 "@internal/cli@workspace:packages/cli-internal":
   version: 0.0.0-use.local
   resolution: "@internal/cli@workspace:packages/cli-internal"