[RFE] Allow user to specify custom scrape config

[surajssd]

Prometheus Operator allows use to specify additional scrape config but it is not exposed at lokoctl level. So expose it using appropriate variables.

---

Right now the user can do the following:

kubectl -n monitoring patch prometheus prometheus-operator-kube-p-prometheus --type merge --patch '{"spec":{"additionalScrapeConfigs":{"name":"scrape-config","key":"scrape.yaml"}}}'
cat > /tmp/scrape.yaml <<EOF
- job_name: "kubernetes-nodes"
  scheme: https
  tls_config:
    ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
  authorization:
    credentials_file: /var/run/secrets/kubernetes.io/serviceaccount/token
  kubernetes_sd_configs:
    - role: node
  relabel_configs:
    - action: labelmap
      regex: __meta_kubernetes_node_label_(.+)
EOF
kubectl -n monitoring create secret generic scrape-config --from-file=/tmp/scrape.yaml