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/package.json

@@ -33,6 +33,7 @@
   },
   "dependencies": {
     "@backstage/backend-common": "workspace:^",
+    "@backstage/backend-plugin-api": "workspace:^",
     "@backstage/config": "workspace:^",
     "@backstage/plugin-tech-insights-common": "workspace:^",
     "@backstage/types": "workspace:^",