support azure china cloud vault backend endpoint

[harshktpa]

Is your feature request related to a problem? Please describe.
The current implementation of the Azure Key Vault backend only supports the standard Azure environment, which means that users working in Azure China cannot seamlessly access their secrets. This limitation creates friction for users who need to operate in both environments.

Describe the solution you'd like
I would like to enhance the Azure Key Vault backend to support dynamic vault URL generation based on the cloud environment. Specifically, when the AVP_AZ_CLOUD_NAME environment variable is set to azurechina, the backend should use the appropriate URL format (i.e., https://.vault.azure.cn) for accessing secrets.

Describe alternatives you've considered
I considered implementing a separate backend specifically for Azure China, but this would lead to code duplication and maintenance challenges.

Additional context
This feature is critical for organizations that operate in multiple Azure environments, especially those that have compliance or operational requirements to store and manage secrets in Azure China.