Add product 'shelf life' field

[jayaddison]

Is your feature request related to a problem? Please describe.
Many kitchen ingredients are 'perishable' and have a shelf life after which they become less suitable - or in some cases completely unsuitable - for consumption.

It would be useful to model the typical shelf life for each product in the products table so that search results that use highly-perishable products can have a ranking boost applied.

Describe the solution you'd like

• Add a shelf_life time duration field to the products table
• A null value for shelf_life can indicate that it is typically shelf-stable for a long enough time duration that a specific value would not be useful

Additional context
Relates to openculinary/api#9