[ci] Use managed identity for API Scan (#138)

I've configured a new [managed identity][0] (MSI) for API Scan, which
allows us to enable a more modern authentication approach when
running API Scan on the `MAUI-1ESPT` agent pool.

A new `$(ApiScanMAUI1ESPTManagedId)` variable has been configured in
the pipeline settings to pass the app ID for this MSI to the
API Scan task.

[0]: https://ms.portal.azure.com/#@microsoft.onmicrosoft.com/resource/subscriptions/cd4829e2-e38b-43d2-8316-2f2009f36f97/resourcegroups/1esobjects/providers/microsoft.managedidentity/userassignedidentities/maui1esptapiscanidentity/overview

Author: pjcollins

azure-pipelines.yml

@@ -425,7 +425,7 @@ extends:
               softwareVersionNum: $(ApiScanSoftwareVersion)
               toolVersion: Latest
             env:
-              AzureServicesAuthConnectionString: runAs=App;AppId=$(ApiScanClientId);TenantId=$(ApiScanTenant);AppKey=$(ApiScanSecret)
+              AzureServicesAuthConnectionString: runAs=App;AppId=$(ApiScanMAUI1ESPTManagedId)
 
           - task: SdtReport@2
             displayName: Guardian Export - Security Report