Change privates in product-variant service to protected

[troykelly]

Is your feature request related to a problem? Please describe. All the variables in the product-variant class are private, which means it can not be (easily) extended.

Describe the solution you'd like Change them to protected (which is in line with the other services, like product)

Describe alternatives you've considered None

Additional context Also would be a good opportunity to fix Typo - Incorrect js doc for SKU retrieval

[olivermrbl]

Thanks for the contribution! Can I get you to change base branch to develop and rebase?

[troykelly]

No problem @olivermrbl, new PR is #2112

[srindom]

@troykelly - We are moving feature requests to GitHub Discussions to keep better track of the most requested features (using the upvote mechanism) and streamline GitHub Issues around bugs. Could I ask you to re-open your request here: https://github.com/medusajs/medusa/discussions/categories/feature-requests

Thanks!