feat(medusa): Simplify the transaction base service (#2007)

**What**
Simplify the transaction base service.

**How**

In fact, it does not need to be template and reduce the extensibility as the type is internally enforce. Now, the type is deduced by this which can be any derived class.

packages/medusa/src/subscribers/search-indexing.ts

@@ -7,13 +7,13 @@ import { ISearchService } from "../interfaces"
 
 type InjectedDependencies = {
   eventBusService: EventBusService
-  searchService: ISearchService<never>
+  searchService: ISearchService
   productService: ProductService
 }
 
 class SearchIndexingSubscriber {
   private readonly eventBusService_: EventBusService
-  private readonly searchService_: ISearchService<never>
+  private readonly searchService_: ISearchService
   private readonly productService_: ProductService
 
   constructor({