Add ability to set log processing stages per loki destination

[petewall]

This would mean that multiple destinations could be set, each with different rules. I would imagine the primary use would be drop/keep rules:

destinations:
- name: localLoki
  type: loki
  url: loki.default.svc
- name: remoteLoki
  type: loki
  url: remote-loki.example.com
  extraLogProcessingStages: |
    stage.drop {
      expression = ".*debug.*"
    }

In this case, all logs go to localLoki, but only non-debug logs are shipped offsite to remoteLoki