Add inventory UI to manage inventory warning threshold per option

[willopez]

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Issue Description

Currently there is no way to set the "Warn at" threshold level per option, it can only be set at the variant level. This is incorrect, since we are tracking inventory per option and per variant. This causes a problem when determining if a "limited supply" badge should be displayed for an option. Without the "Warn at", level set per option, it's not possible to make that determination. This issue will address this need by adding a "Warn at" input field at the option level, refer to image below for a visual. A background discussion is found here: #4342

Possible Solution

Adding UI to set "Warn at" level per option.

[spencern]

Are we currently using the warn at level from the variant?
Can we imply that the option warn at levels are the same as the level set for variants for now?

[willopez]

@spencern Yes, the parent top level variant's warn at level could be used as a work around. Nat suggested that here: #4342 (review). I am not fond of that solution, cause it will lead to confusion in the future, and it's not correct, given that inventory is tracked at the variant/option level.

[spencern]

@willopez I agree it's not a great solution in the long run. If it's a trivial thing to add to the product option operator panel, let's schedule it for our next sprint.

[willopez]

@spencern I looked at what it would take, and it's like a 5-8 effort.

[spencern]

Thanks @willopez. Not quite trivial, let's hold off on this for now and plan to solve it when we revisit product management.