feat(medusa): Retrieve (service + controller) a product category (#3004)

What:

Introduces a store endpoint to retrieve a product category

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-967

.changeset/empty-beers-pull.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/medusa": patch
+---
+
+feat(medusa): create a store endpoint to retrieve a product category