Adds weight to Shopify Variant

[hackjiyi]

weight

[ryankazokas]

This change looks good, but please read our contribution docs:
https://github.com/ChannelApe/shopify-sdk/blob/master/CONTRIBUTING.md

We need test coverage on new code that is added. Should be fairly simple to add for this change.

Also, the target branch should be the develop branch for new changes

[ryankazokas]

@hackjiyi any chance you can update the above changes this week? We are working on the next release and can include this in there.

One other thing that might be useful for this PR, would be to add the Weight Units attribute. Looks like not all variants have the same interpretation of weight.
https://shopify.dev/docs/admin-api/rest/reference/products/product-variant

The unit of measurement that applies to the product variant's weight. If you don't specify a value for weight_unit, then the shop's default unit of measurement is applied. Valid values: g, kg, oz, and lb.