Fail gracefully when Redis not available

### Component

Graphql Engine (caching)

### Is your proposal related to a problem?

Customers report to me that:

- If redis goes down, Hasura goes down. They would like more fault tolerance and graceful failures
- Sometimes headers are desired for higher up in the ecosystem e.g. CDN and Redis is not required

### Describe the solution you'd like

A situation where if Redis is not up/online, Hasura continues to serve requests, with appropriate headers (albeit with a header indicating the result is not cached in Redis).

This would achieve both goals vis-a-vis redis being down not impacting hasura and cache headers being able to be used without redis.

A backport of this functionality to v2 would also be desirable for existing customers.

### Describe alternatives you've considered

N/A