Allow patching resource before updating Grafana

[Baarsgaard]

Is your feature request related to a problem? Please describe.
A way to somehow patch/update resources before applying the resource to a Grafana instance.

Describe the solution you'd like
Potentially: yq/jq paths or the Kubernetes Downward API as a way to template any field on a resource.

Maybe extending the valuesFrom field to support more options?

Like:

• Replacing part of the url in a datasource.
• Editing the title of a folder/dashboard depending on the instance name/namespace.
• Reference values in ConfigMaps/Secrets from the target instance namespace.

Describe alternatives you've considered
Managing multiple copies of the same CRs with different instanceSelector values.

Existing solutions
Generating resources with Grafana Tanka, kubecfg, Terraform, or other tools. But they often lack the information available in Kubernetes.

Similar issues and discussions:

• Support jsonnet libraries #2161
• Datasource with allowCrossNameSpaceImport get cm/secret in ns with grafana #1861
• GrafanaDashboard should support substitution from GrafanaDatasource name #1684
• GrafanaDashboard syncing from url and replacing variables on the go #1760
• Deprecation of Jsonnet dashboard type #2171