feat(tech-insights): add plugin for new backend system

A new backend plugin for the tech-insights backend
was added and exported as `default`
as well as extension points to be used by modules.

The plugin can be used like

```ts title="packages/backend/src/index.ts"
backend.add(import('@backstage/plugin-tech-insights-backend'));
```

Fact retrievers will be added
(built-in or through the extension point),
but only registered if the user adds a configuration for it
(cadence, etc.).

Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

plugins/tech-insights-node/src/extensionPoints.ts

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { createExtensionPoint } from '@backstage/backend-plugin-api';
+import { CheckResult } from '@backstage/plugin-tech-insights-common';
+import { FactRetriever } from './facts';
+import { FactCheckerFactory, TechInsightCheck } from './checks';
+
+/**
+ * @public
+ */
+export interface TechInsightsFactRetrieversExtensionPoint {
+  addFactRetrievers(factRetrievers: Record<string, FactRetriever>): void;
+}
+
+/**
+ * An extension point that allows other plugins or modules to add fact retrievers.
+ *
+ * @public
+ */
+export const techInsightsFactRetrieversExtensionPoint =
+  createExtensionPoint<TechInsightsFactRetrieversExtensionPoint>({
+    id: 'tech-insights.fact-retrievers',
+  });
+
+/**
+ * @public
+ */
+export interface TechInsightsFactCheckerFactoryExtensionPoint {
+  setFactCheckerFactory<
+    CheckType extends TechInsightCheck,
+    CheckResultType extends CheckResult,
+  >(
+    factory: FactCheckerFactory<CheckType, CheckResultType>,
+  ): void;
+}
+
+/**
+ * An extension point that allows other plugins or modules to set a FactCheckerFactory.
+ *
+ * @public
+ */
+export const techInsightsFactCheckerFactoryExtensionPoint =
+  createExtensionPoint<TechInsightsFactCheckerFactoryExtensionPoint>({
+    id: 'tech-insights.fact-checker-factory',
+  });

plugins/tech-insights-node/src/index.ts

@@ -15,5 +15,6 @@
  */
 
 export * from './checks';
+export * from './extensionPoints';
 export * from './facts';
 export * from './persistence';