feat(ecommerce): add admin module

[amitHarwani]

Closes #157

Features Implemented for an Admin User:

1. View Categories, Add a category, Update a category, Delete a category.
2. View Products, Add a product, Update a product, Delete a product.
3. View Coupons, Add a coupon, Update a coupon, Delete a coupon.
4. View Orders, Update Order Status

[wajeshubham]

Hi @amitHarwani ,

Can you please share live link of the working application with freeapi as a backend? also the branch has conflicts with original branch. Kindly address.