Feature - Add inventory level SKU

[supercid]

Description

Amend SKU entity to add product inventory level.

Related Issue

Closes #520

Motivation and Context

We have the field available on Playcart, but we are not sending the data.

How Has This Been Tested?

Built product, data has been checked on Playcart payload and UI.
Product has been correctly sanitized and there's no inventory level data on the tagging.

Documentation:

N/A

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• I have assigned the correct milestone or created one if non existent.
• I have correctly labeled this pull request.
• I have linked the corresponding issue in this description.
• I have updated the corresponding Jira ticket.
• I have requested a review from at least 2 reviewers
• I have checked the base branch of this pull request
• I have checked my code for any possible security vulnerabilities
• I have updated the SDK version.