Allow Variant to check stock by stock_location

[MadelineCollier]

Description
Quantifier already has the ability to restrict stock checks to certain locations. This change just exposes that ability at the Variant level. This allows applications with multiple stock locations to check variant stock for a specific location without explicitly having to instantiate a new Quantifier with the stock location.

These changes allow both Variant#total_on_hand and Variant#can_supply? to optionally take a stock_location

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[jarednorman]

This is a nice additive change and most of the stores I've worked on that needed to handle multiple locations have done something like this. I like.

[kennyadsl]

Thanks for the improvement. I'd wait for 3.1 before merging this though, adding this PR to the right milestone to track this. 🙏

[spaghetticode]

@MadelineCollier thank you 🎉