backend-app-api: deprecate callback form of backend.add(...)

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

.changeset/three-lizards-crash.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-app-api': patch
+---
+
+The ability to install backend features in callback form (`() => BackendFeature`) has been deprecated. This typically means that you need to update the installed features to use the latest version of `@backstage/backend-plugin-api`. If the feature is from a third-party package, please reach out to the package maintainer to update it.

packages/backend-app-api/api-report.md

@@ -58,9 +58,16 @@ export interface Backend {
   add(
     feature:
       | BackendFeature
+      | Promise<{
+          default: BackendFeature;
+        }>,
+  ): void;
+  // @deprecated (undocumented)
+  add(
+    feature:
       | (() => BackendFeature)
       | Promise<{
-          default: BackendFeature | (() => BackendFeature);
+          default: () => BackendFeature;
         }>,
   ): void;
   // (undocumented)

packages/backend-app-api/src/wiring/types.ts

@@ -25,11 +25,18 @@ import {
  * @public
  */
 export interface Backend {
+  add(feature: BackendFeature | Promise<{ default: BackendFeature }>): void;
+  /**
+   * @deprecated The ability to add features defined as a callback is being
+   * removed. Please update the installed feature to no longer be defined as a
+   * callback, typically this means updating it to use the latest version of
+   * `@backstage/backend-plugin-api`. If the feature is from a third-party
+   * package, please reach out to the package maintainer to update it.
+   */
   add(
     feature:
-      | BackendFeature
       | (() => BackendFeature)
-      | Promise<{ default: BackendFeature | (() => BackendFeature) }>,
+      | Promise<{ default: () => BackendFeature }>,
   ): void;
   start(): Promise<void>;
   stop(): Promise<void>;