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>

packages/backend-internal/package.json

@@ -0,0 +1,28 @@
+{
+  "name": "@internal/backend",
+  "version": "0.0.1",
+  "backstage": {
+    "role": "node-library",
+    "inline": true
+  },
+  "private": true,
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/backstage/backstage",
+    "directory": "packages/backend-internal"
+  },
+  "license": "Apache-2.0",
+  "sideEffects": false,
+  "main": "src/index.ts",
+  "types": "src/index.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "lint": "backstage-cli package lint",
+    "test": "backstage-cli package test"
+  },
+  "devDependencies": {
+    "@backstage/cli": "workspace:^"
+  }
+}