Move ExtensionFactoryMiddleware from frontend-plugin-api to frontend-app-api

The `ExtensionFactoryMiddleware` type is only used by the app-level wiring
and has been moved to `@backstage/frontend-app-api` as its canonical location.

The type is preserved in `@backstage/frontend-plugin-api` with a `@deprecated`
tag pointing to the new location. All internal usages have been updated to
import from the new location.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

.changeset/move-extension-factory-middleware-app-api.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+Added the `ExtensionFactoryMiddleware` type as a public export.

.changeset/move-extension-factory-middleware-plugin-api.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-plugin-api': patch
+---
+
+Deprecated the `ExtensionFactoryMiddleware` type, which has been moved to `@backstage/frontend-app-api`.