feat(medusa): List (service + controller) product categories #3004 (#3023)

**What:**

Introduces a store endpoint to retrieve a list of product categories

**Why:**

This is part of a greater goal of allowing products to be added to multiple categories.

**How:**

- Creates an endpoint in store routes

RESOLVES CORE-968

packages/medusa/src/models/product-category.ts

@@ -17,6 +17,8 @@ import {
 @Entity()
 @Tree("materialized-path")
 export class ProductCategory extends SoftDeletableEntity {
+  static treeRelations = ["parent_category", "category_children"]
+
   @Column()
   name: string