feat: add Azure Key Vault secret operations

[atharva-nagane]

Context

RustCloud has Azure auth and Blob Storage but no Key Vault — the Azure
service for secrets, keys, and certificates. This is the Azure equivalent
of AWS Secrets Manager and completes the security category for Azure.

Operations to implement

Function	Description
set_secret	Create or update a secret by name
get_secret	Retrieve the current or a specific version
delete_secret	Soft-delete a secret
list_secrets	List all secret names in the vault
get_secret_versions	List all versions of a secret

Acceptance criteria

• AzureKeyVault struct reading AZURE_KEYVAULT_URL and AZURE_KEYVAULT_TOKEN
• All 5 operations via Key Vault REST API v7.4
• Wired in main.rs and tests/mod.rs
• 5 tests + example doc at examples/azure/security/key_vault.md
• README updated (Azure Security row)