feat: allow overriding default techdocs preparers

a new extension point `TechdocsPreparerExtensionPoint` allows
registering custom preparers for the techdocs plugin.

closes #21952

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/techdocs-node/src/extensions.ts

@@ -15,7 +15,7 @@
  */
 import { createExtensionPoint } from '@backstage/backend-plugin-api';
 import { DocsBuildStrategy } from './techdocsTypes';
-import { TechdocsGenerator } from './stages';
+import { PreparerBase, RemoteProtocol, TechdocsGenerator } from './stages';
 
 /**
  * Extension point type for configuring Techdocs builds.
@@ -54,3 +54,22 @@ export const techdocsGeneratorExtensionPoint =
   createExtensionPoint<TechdocsGeneratorExtensionPoint>({
     id: 'techdocs.generator',
   });
+
+/**
+ * Extension point type for configuring a custom Techdocs preparer
+ *
+ * @public
+ */
+export interface TechdocsPreparerExtensionPoint {
+  registerPreparer(protocol: RemoteProtocol, preparer: PreparerBase): void;
+}
+
+/**
+ * Extension point for configuring a custom Techdocs preparer
+ *
+ * @public
+ */
+export const techdocsPreparerExtensionPoint =
+  createExtensionPoint<TechdocsPreparerExtensionPoint>({
+    id: 'techdocs.preparer',
+  });

plugins/techdocs-node/src/index.ts

@@ -26,6 +26,8 @@ export * from './techdocsTypes';
 export {
   techdocsBuildsExtensionPoint,
   techdocsGeneratorExtensionPoint,
+  techdocsPreparerExtensionPoint,
   type TechdocsBuildsExtensionPoint,
   type TechdocsGeneratorExtensionPoint,
+  type TechdocsPreparerExtensionPoint,
 } from './extensions';