Get Dashboard JSON from external store

[abacus3]

The current API implementation requires the specification of the entire Grafana Dashboard JSON to be provided in the terraform resource property config_json directly.

This works well with Terraform per se. Nevertheless, other tools using this provider, such as the crossplane provider-grafana, may encounter issues with limitations in other ecosystems, such as the k8s object size limit. (which in combination with the well-known last-applied k8s annotation results in only half of the acual object size to be usable)

In order to support large dashboards with k8s crossplane provider-grafana, I'd like to propose the following feature: Allow dashboards to be loaded from external data stores, such as git, s3, filesystem etc.

Additional clients and config of those for the external stores would be required here. Credentials for the external storage system should be provided in the resource definition, not in the provider section, so that multiple backend systems can be supported independently.