Add certificate-based Service Principal authentication to the Azure
provider. AzureProvider accepts a certificate (base64 content or file
path) and authenticates via azure.identity.CertificateCredential,
mirroring the M365 provider flow. Includes CLI flags
(--certificate-auth, --certificate-content, --certificate-path),
key-pair validation for PEM and PKCS#12 bundles, and unit tests.