feat: add new fields to inventory model

[Jonne]

Hi everyone,

First of all, great job on the Commerce Layer Terraform provider, makes our job a lot easier 👏

I hope you don't mind I took the liberty to create this PR for #111 . I thought it was a nice way to learn about the workings of the project.

This PR adds some fields that were recently introduced to the Inventory Model.

• manual_stock_decrement
• stock_reservation_cutoff
• put_stock_transfers_on_hold

https://docs.commercelayer.io/core/api-reference/inventory_models/object

[demeyerthom]

LGTM