feat(medusa): added admin create endpoint for nested categories (#2985)

What:

Introduces an admin endpoint that allows a user to create a product category

Why:

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

How:

- Creates a route on the admin scope to create category
- Creates a method in product category services to create a category

RESOLVES CORE-958

.changeset/hungry-starfishes-count.md

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+feat(medusa): added admin create endpoint for product categories