External cache storage

[guicassolato]

#247 delivered the first step of runtime evaluator caching, i.e. in-memory caching. Next step is to make possible for users to extend/move this type of caching to an external storage (e.g. Redis database).

Benefits include:

1. Better management of cache size and cache resources, working around the constraints imposed at the level of the Authorino deployment (to protect the instance);
2. Sharing the cache between multiple Authorino instances and deployments.

A possible API could look like the following:

spec:
  authorization:
  - name: my-policy
    cache:
      key: { valueFrom: context.request.http.headers.authorization }
      ttl: 120
      external:
        url: redis://my-redis/db