fix: don't use dashboard cache if url is different

[weisdd]

Description

Currently, dashboard cache is still being used even if URL changes. The PR addresses that.

Other changes:

• fix: lift restriction on having both url and grafanacom defined. Since there's a fallback mechanism in place that handles such case, there's not much sense in having the restriction;
• chore: for code simplification, moved from *metav1.Duration to metav1.Duration where the former was not really needed (GrafanaDashboardStatus, GrafanaSpec);
• fix: for unknown reasons, NewDashboardPipeline would fallback to 24h instead of 0(=infinity) if dashboard.Spec.ContentCacheDuration is not defined. Removed that;
• chore: moved cache extraction from obtainJson() to loadDashboardFromURL(). - I think that location makes more sense.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

PR

I think tests should already offer a comprehensive picture.

Extra notes

Ideally, we should have an automated e2e-test for testing the caching mechanism. For now, something like this would work:

1. Save a dashboard to a local disk;
2. Start caddy:

caddy file-server --listen :2015 --access-log

3. Reference the dashboard with your IP:

apiVersion: integreatly.org/v1alpha1
kind: GrafanaDashboard
metadata:
  name: grafana-dashboard-from-url
  labels:
    app.kubernetes.io/instance: grafana-operator
spec:
  url: http://192.168.64.1:2015/victoriametrics-cluster_rev2.json

Depending on the settings, you should start seeing records in the access log.

Options:

• contentCacheDuration: 20s in GrafanaDashboard spec;
• dashboardContentCacheDuration: 1m in Grafana spec;
• And no updates at all if URL is the same and the timer is not set anywhere.

[NissesSenap]

I think this looks good but I would love a second par of eyes from ether @pb82 or @HubertStefanski

[pb82]

LGTM, a different URL will result in an empty result from the cache leading to a re-fetch of the content.