make rulerAPIPath configurable to support mimir

[janwillies]

make the rulerAPIPath configurable to support mimir, which has a different path than cortex

example provider-config for mimir:

apiVersion: cortex.crossplane.io/v1alpha1
kind: ProviderConfig
metadata:
  name: mimir
spec:
  tenantId: example-tenant
  address: http://mimir.default.svc:9009
  rulerAPIPath: /prometheus/config/v1/rules

note that this depends on cortexproject/cortex-tools#19 and thus replaces this dependency with a fork here: https://github.com/janwillies/cortex-tools/tree/configure-rules-endpoint

fixes #4