Use descriptive rate limit instead of delay in resource

[isra17]

Replace default_delay by rate_limit that can be a rate by window. This way we could support API that support a given rate by month and bursting this rate in a few days instead of spreading the load through a month. We might want to support many rate limit so we can set 1000 per month and 1 per 5 seconds.

cc @mlefebvre

[isra17]

I think something like:

---
apiVersion: saturn.flared.io/v1alpha1
kind: SaturnResource
metadata:
  name: example-api-key-1
spec:
  type: example.resources.TestApiKey
  data:
    key: example-api-key-1
  rate_limits:
    - 1000 per month
    - 1 per second

would work well. Note that we still need to support ResourceUsed with a delay.