Units creation endpoint

[conradbekondo]

Add an API endpoint which takes a product's ID and then creates a unit for that product with the specified configuration for the unit.

• The endpoint should receive POST requests only.
• The endpoint should have a required parameter: productId which should represent the ID of an existing product in the system.
• The endpoint should take as its body, a collection of configurations for units to be created.
• The endpoint should reject creating units if there is already such a unit with the same name for that particular product.
• The endpoint should reject creating all the units should the creation of one unit fail, hence the operation should be transactional.