make available the requests_cache backend configuration

Created by: al-mihai

Hi,

Thank you for Flaat!

While using it with a Flask application on Azure AppService, we encountered an Azure bug: https://github.com/Azure/azure-sdk-for-python/issues/6503. This is because Flaat uses requests_cache with a default configuration using SQLite, which is the object of that bug. Can you provide a Flaat API to set/configure the backend for requests_cache? (I can confirm it works on Azure with memory configured as backend)

Cheers, Alexandru

PS I can do a Pull Request, if you don't have the time to add this functionality