Add checks if a user is already in the org due to auto_assign_org_id

[PovilasV1]

related to https://github.com/grafana/terraform-provider-grafana/issues/1750

If the user is added to org by auto_assign_org_id, the provider doesn't see it and tries to add it again, skipping the role update.

There might be a better solution, but I don't have experience with Terraform providers and Golang.

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[Duologic]

@grafana/identity-squad Is this a reasonable change? Should we move forward with this for issue #1750 ?