Azure AD Workload Identity authentication support

[kostrse]

This PR adds support for Azure AD Workload Identity which is used for access to AAD service identities in Kubernetes.

This allows users who host Grafana in Kubernetes be able to use Workload Identities as alternative to Managed Identities which are not directly accessible by pods in Kubernetes.

Details on Azure AD Workload Identity:

• https://azure.github.io/azure-workload-identity
• https://github.com/Azure/azure-workload-identity

The newly introduced credentials type is AzureWorkloadIdentityCredentials.

Intended to be serialized in datasource JSON as:

{
    "azureCredentials" {
        "authType": "workloadidentity"
    }
}

The credentials object doesn't provide any configuration options on the datasource side, but on the level it will be possible to override Tenant ID, Client ID, and Token File path.

Proposed configuration settings in Grafana config (not part of this PR):

[azure]
workload_identity_enabled = true

# These settings override default Workload Identity and normally not required
workload_identity_tenant_id =
workload_identity_client_id =
workload_identity_token_file =

[aangelisc]

LGTM 🎉