feat: add Azure Blob Storage as a third cloud storage provider

[navneet-patteri]

• Implements AzureBackend (backends/azure_backend.py) conforming to the existing CloudStorageBackend abstract interface — upload, delete, signed
  URL generation, public URL, and connection test
• Adds azure_account_name, azure_private_container_name, azure_public_container_name, and azure_account_key fields to the Cloud Storage Configuration doctype; storage provider Select now includes Azure Blob Storage
• Supports two auth modes: Storage Account Key (encrypted at rest, same pattern as S3 secret) and Managed Identity via
  DefaultAzureCredential — leave the key blank and azure-identity handles the credential chain
• Private file signed URLs use SAS tokens generated with the account key or a User Delegation Key (Managed Identity path — requires Storage
  Blob Delegator role on the storage account)
• Public files are served via direct blob URLs; public container must have Blob-level public access enabled in the Azure portal
• Azure blob URLs (*.blob.core.windows.net) added to _is_cloud_file_url in controller.py to prevent re-upload of already-migrated files
• azure-storage-blob>=12.0.0 and azure-identity>=1.0.0 added to requirements.txt
• README updated to document the new provider, its configuration fields, and the Managed Identity setup requirements