Add 'grafana_synthetic_monitoring_secret' resource

[rfhold]

Feature Request

https://grafana.com/docs/grafana-cloud/testing/synthetic-monitoring/create-checks/manage-secrets/

It would be nice to be able to manage the secrets used in synthetic checks though this provider

[mem]

• Ask operator experience squad about their status re: Terraform support.

In principle opex squad is in charge of delivering this. Synthetic Monitoring does provide a custom UI, so we need to figure out how to make that match with the existing Terraform support.

This needs some investigation before we can share more details.

[mem]

Assigning to @d0ugal for investigation.