feat(recommend): add "Looking Similar" model

[raed667]

Implement Recommend looking-similar model.

Beyond the model name, everything else should work exactly as other recommend models such as related products.

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[raed667]

@shortcuts indeed we should this to the generated clients as well 👍

@dhayab No beta/RC needed, once merged this should probably be simply released under 4.19.0

[shortcuts]

@shortcuts indeed we should this to the generated clients as well 👍

cool! let me know if you need help contributing to the specs