Add DefaultAzureCredential auth mechanism support

The current ObjectStore API doesn't support authenticating to Azure using [DefaultAzureCredential](https://learn.microsoft.com/en-us/dotnet/api/azure.identity.defaultazurecredential?view=azure-dotnet). This is a limitation in AKS environments where `managed-identities` are not configured.

Even if the environment is configured to use the app Service Principal with variables `AZURE_CLIENT_ID`, `AZURE_TENANT_ID`, and `AZURE_CLIENT_SECRET` the barman-cloud client attempts to authenticate using the managed-identity and fails if not available.

Start with the ObjectStore API and the barman-cloud client authentication path, then review the DefaultAzureCredential documentation linked in the issue. Confirm how AZURE_CLIENT_ID, AZURE_TENANT_ID, and AZURE_CLIENT_SECRET are handled, and consider the work complete when configured service-principal credentials authenticate successfully without requiring a managed identity.

Written by the indexing model from the issue text.