Backend fallback: Fallback option when backend is unavailable

[rprodhomme]

Feature description

Use case

Is there a specific goal that this would help you accomplish, or can you provide any other context about how you would like to use this feature?
I'm using requests-cache with a redis backend and in case of failure of redis (for whatever reason) I'd like to bypass the cache and do the call instead. Currently the call fails as it can't reach the backend and I haven't seen any option to change this behaviour.

An option to do so would be nice (like having an in memory cache as a fallback backend for instance, if we want to avoid doing the call without cache).

Workarounds

Is there an existing workaround to accomplish this?
I guess this could be implemented as a CustomCache backend but I'd rather see it added in the lib.

Plan to implement

Are you interested in submitting a PR to implement this?
That could be an option if we agree on the feature and its design.