Retry support for azurerm_virtual_network_peering

[tiwood]

This PR introduces retry support (Create/Update) for azurerm_virtual_network_peering.

This is required as sometimes one of the virtual networks involved in the peering is not completely provisioned (just created or just updated), in which case the following error is returned from the API:

{
	"error": {
		"code": "ReferencedResourceNotProvisioned",
		"message": "Cannot proceed with operation because resource /subscriptions/subscriptionId/resourceGroups/resourceGroupName/providers/Microsoft.Network/virtualNetworks/vNetName used by resource /subscriptions/subscriptionId/resourceGroups/resourceGroupName/providers/Microsoft.Network/virtualNetworks/virtualNetworkName/virtualNetworkPeerings/vnetPeeringName is not in Succeeded state. Resource is in Updating state and the last operation that updated/is updating the resource is PutSubnetOperation.",
		"details": []
	}
}

This fixes #2605

[tombuildsstuff]

hey @tiwood

Thanks for this PR - this LGTM 👍

[tombuildsstuff]

Tests pass:

[ghost]

This has been released in version 1.28.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
	version = "~> 1.28.0"
}
# ... other configuration ...

[ghost]

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