azurerm_storage_management_policy - Add existance check

[magodo]

The concern is whether the lock here is necessary, as if that is the case, then I believe almost all of the existing resources should add that. But at least, it is needed for the use case in #18583.

Fix #18583

Test

💤  TF_ACC=1 go test -v -timeout=20h -run='TestAccStorageManagementPolicy_requiersImport' ./internal/services/storage
=== RUN   TestAccStorageManagementPolicy_requiersImport
=== PAUSE TestAccStorageManagementPolicy_requiersImport
=== CONT  TestAccStorageManagementPolicy_requiersImport
--- PASS: TestAccStorageManagementPolicy_requiersImport (152.71s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/storage       152.721s

[mbfrahry]

Hey @magodo, this change looks good minus what you called out with the locks. We should do a separate PR for that so we can get this in quicker

[mbfrahry]

I'm not sure we should go this route quite yet. We should do a separate PR with it's own test to confirm that this will fix that issue.

[magodo]

Yeah, I've removed the locks.

[stephybun]

Thanks @magodo LGTM 👍

[github-actions]

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