Define role per organization in the User CR

[angelbarrera92]

It would be nice to add a new feature in the User CRD to allow defining a different role other than viewer.

---
apiVersion: grafana.k8spin.cloud/v1
kind: User
metadata:
  name: angelbarrera92
  namespace: monitoring
spec:
  email: angelbarrera92@yopmail.com
  organizations:
    - k8spin.cloud

Could be nice to have a new structure in the organizations list other than a list of strings:

---
apiVersion: grafana.k8spin.cloud/v1
kind: User
metadata:
  name: angelbarrera92
  namespace: monitoring
spec:
  email: angelbarrera92@yopmail.com
  organizations:
    - name: k8spin.cloud
      role: admin
    - name: example.com
      role: viewer
    - name: default.io

If no role is set, viewer should be the default one. Consider that a User could be viewer in an organization and admin in a different one.