Add Azure authentication module (MSI, Workload Identity, Service Principal) for azure_blob output plugin

[zshuang0316]

Is your feature request related to a problem? Please describe.
out_azure_blob currently only supports shared key and SAS token authentication. Shared key is not a recommended or secure approach as it grants full account access. SAS token has an expiration time, requiring manual rotation and operational overhead. We hope to leverage OAuth/Managed Identity for authentication instead.

Describe the solution you'd like

Add a common Azure authentication module (flb_azure_auth) to Fluent Bit core that supports:

• System-assigned and user-assigned Managed Identity (MSI) via IMDS endpoint
• Workload Identity authentication with federated tokens
• Service Principal authentication with client credentials (OAuth2 client_credentials flow)

Integrate this into out_azure_blob alongside the existing methods (fully backward compatible), with new config
parameters:

• tenant_id, client_id, client_secret — for Service Principal
• workload_identity_token_file — for Workload Identity
• client_id = system — for system-assigned Managed Identity

Also migrate out_azure_kusto to use the same common module, removing its local duplicate azure_msiauth.c.

A working implementation is available at: https://github.com/zshuang0316/fluent-bit/tree/azure-msi-auth

Describe alternatives you've considered

Additional context

[patrick-stephens]

Possibly related to #7525