[Woo POS][Products Search] Add POSProduct entity

[joshheald]

Add a minimal POSProduct entity, containing the fields we need for search, display, and adding to a cart in POS

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.

[iamgabrielma]

@joshheald Is this one still valid, or can be closed? As we have POSSimpleProduct, POSVariation, POSVariableParentProduct, ... I'm unsure if covers the same case as the POSProduct entity.