[FEATURE] Don't destroy workspace binding completely if it's the last binding for an object

[alexott]

When destroying the workspace binding we may need to check if it's the last binding for a current UC object, and if it's then don't destroy it until UC object has isolation mode changed to OPEN.

Implementing this will prevent errors that UC object isn't available in the current workspace, like this:

databricks_workspace_binding.service_cred: Destruction complete after 2s
databricks_credential.credential: Destroying... [id=aott-db-service-isolated]
╷
│ Error: cannot delete credential: Credential 'aott-db-service-isolated' is not accessible in current workspace
│
│
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