Implement spec.uid for GrafanaDatasource

[Baarsgaard]

I noticed the editable field actually does work, it is not used by the Operator, but is is part of the serialized payload and will configure the resulting datasource.
The same was true for OrgId

Manual Testing

make start-kind
kind export kubeconfig --name kind-grafana
kubectl delete grafana-operator --all
kubectl apply -f tests/example-resources.yaml
kubectl port-forward svc/grafana-testdata-service 3000 >/dev/null &
# Open http://localhost:3000/d/testdata-dash/new-dashboard
# username: root
# password: secret

kubectl apply -f tmp.yml
# See that the dashboard panel is populated with 2 new datasources
# One with a generated UID and one with a defined UID
# Repeatedly deleting and applying tmp.yml should result in a new UID for one of the datasources every time.

tmp.yml

---
apiVersion: grafana.integreatly.org/v1beta1
kind: GrafanaDatasource
metadata:
  name: spec-uid
spec:
  resyncPeriod: 3s
  instanceSelector:
    matchLabels:
      test: "testdata"
  uid: spec-uid
  datasource:
    orgId: 1
    isDefault: true
    name: spec-uid
    type: grafana-testdata-datasource
    access: proxy
    basicAuth: false
---
apiVersion: grafana.integreatly.org/v1beta1
kind: GrafanaDatasource
metadata:
  name: metadata-uid
spec:
  resyncPeriod: 3s
  instanceSelector:
    matchLabels:
      test: "testdata"
  datasource:
    orgId: 1
    isDefault: true
    name: metadata-uid
    type: grafana-testdata-datasource
    access: proxy
    basicAuth: false