Enable getting more than one property at a time

[alexgarel]

Problem

Currently we can retrieve all properties of a product or a specific key

But maybe we want 2 or 3 precise keys for a product.

Proposed solution

Add a keys optional parameter to /product/{product} route that enable listing keys to fetch.

I propose keys to be separated by a column (or whatever is easier to support with OpenAPI/FastAPI).

[kirtanchandak]

working on this