Azure Stack HCI Bicep: Add ARB Subscription Role Assignment

[mbrat2005]

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• Added ARB SP role assignment, parameter, and docs

[mbrat2005]

@hillieton The secret is a secret credential for an Entra app registration and is currently a requirement for ASHCI Resource Bridge deployment. The same as described here: https://learn.microsoft.com/en-us/azure-stack/hci/deploy/deployment-azure-resource-manager-template#create-a-service-principal-for-arb