changesets: add changesets for feature loaders

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

.changeset/curvy-pillows-joke.md

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+---
+'@backstage/backend-app-api': patch
+---
+
+Added support for the latest version of `BackendFeature`s from `@backstage/backend-plugin-api`, including feature loaders.

.changeset/cyan-shrimps-push.md

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+---
+'@backstage/backend-test-utils': patch
+---
+
+Internal updates to support latest version of `BackendFeauture`s from `@backstage/backend-plugin-api`.

.changeset/khaki-lamps-peel.md

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+---
+'@backstage/backend-plugin-api': patch
+---
+
+Added `createBackendFeatureLoader`, which can be used to programmatically select and install backend features.
+
+A feature loader can return an list of features to be installed, for example in the form on an `Array` or other for of iterable, which allows for the loader to be defined as a generator function. Both synchronous and asynchronous loaders are supported.
+
+Additionally, a loader can depend on services in its implementation, with the restriction that it can only depend on root-scoped services, and it may not override services that have already been instantiated.
+
+```ts
+const searchLoader = createBackendFeatureLoader({
+  deps: {
+    config: coreServices.rootConfig,
+  },
+  *loader({ config }) {
+    // Example of a custom config flag to enable search
+    if (config.getOptionalString('customFeatureToggle.search')) {
+      yield import('@backstage/plugin-search-backend/alpha');
+      yield import('@backstage/plugin-search-backend-module-catalog/alpha');
+      yield import('@backstage/plugin-search-backend-module-explore/alpha');
+      yield import('@backstage/plugin-search-backend-module-techdocs/alpha');
+    }
+  },
+});
+```