[azure-eventhub] Add support for Workload Identity

## Context

The azure-eventhub input originally used connection strings as the only authentication method. Since introducing processor v2 with the modern Event Hubs SDK, we now support multiple authentication options through the `azidentity` module.

**Workload Identity** is the recommended authentication method for Kubernetes workloads on Azure Kubernetes Service (AKS) and other Kubernetes distributions. It replaces the deprecated pod-managed identity approach and enables secure, secret-free authentication using OpenID Connect (OIDC) federation with Microsoft Entra ID.

Key characteristics:
- **Zero secrets management** - No credentials stored in cluster or configuration
- **Kubernetes-native** - Uses Service Account Token Volume Projection
- **Multi-cloud support** - Works on AKS, Azure Arc-enabled clusters, and other Kubernetes distributions
- **Industry standard** - Currently used by Azure Monitor for Prometheus remote write authentication
- **Enhanced security** - Short-lived tokens with automatic rotation

The azure-eventhub input uses the Azure Event Hubs SDK based on the `azidentity` module, which supports `WorkloadIdentityCredential` out of the box.

## Goal

TBD

## Impact

By adding Workload Identity support, we enable:

- Enhanced Security - Eliminates credential storage in Kubernetes clusters, reducing attack surface and meeting compliance requirements for secret-free deployments
- Simplified Operations - No certificate rotation, key management, or secret synchronization across multiple clusters
- Broader Adoption - Aligns with Microsoft's recommended approach for AKS workloads, making the azure-eventhub input the standard choice for Kubernetes-based log collection
- Multi-cloud Flexibility - Supports Azure Arc-enabled Kubernetes clusters running on-premises or in other clouds, expanding deployment options
- Better Developer Experience - Follows the same pattern as Azure Monitor's Prometheus remote write, providing consistency across Azure observability tools