userAssignedIdentities providerID value format unexpectedly changed in CAPZ v1.9.0

[dthorsen]

/kind bug

What steps did you take and what happened:
Tried to bootstrap a new cluster with an AzureMachineTemplate that worked in CAPZ v1.7.0. The azurejson_machinetemplate_controller failed to create a secret for the azure json. The following error was emitted in the logs:

E0530 20:42:02.758002       1 controller.go:329]  "msg"="Reconciler error" "error"="failed to get user-assigned identity ClientID: invalid resource ID: resource id 'azure:///subscriptions/<snip>/resourcegroups/<snip>/providers/Microsoft.ManagedIdentity/userAssignedIdentities/<snip>' must start with '/'" "AzureMachineTemplate"={"name":"<snip>","namespace":"<snip>"} "controller"="azuremachinetemplate" "controllerGroup"="infrastructure.cluster.x-k8s.io" "controllerKind"="AzureMachineTemplate" "name"="<snip>" "namespace"="<snip>" "reconcileID"="<snip>"

If I use an AzureMachineTemplate that has a userAssignedIdentity providerID value that starts with /subscriptions instead of azure:///subscriptions the error goes away and things work as expected. However this is not how the field is documented to be used here.

It looks like this bug was introduced because the behavior of the new ParseResourceID function from the new arm package is not fully compatible with the ParseResourceID function from the autorest package that was used previously.
Old function: https://github.com/Azure/go-autorest/blob/autorest/v0.11.29/autorest/azure/azure.go#L215-L217
New function: https://github.com/Azure/azure-sdk-for-go/blob/sdk/azcore/v1.6.0/sdk/azcore/arm/internal/resource/resource_identifier.go#L64C6-L71

What did you expect to happen:
Existing, working AzureMachineTemplate yaml should work when upgrading CAPZ from v1.7.0 to v1.9.2.

Anything else you would like to add:
It looks like the breaking change was introduced in #3337.

Environment:

• cluster-api-provider-azure version: v1.9.2
• Kubernetes version: (use kubectl version): v1.21.14
• OS (e.g. from /etc/os-release): Ubuntu 20.04

[CecileRobertMichon]

cc @mboersma

[jackfrancis]

/assign @mboersma