VAULT-14644 Add support for Azure WIF auth to auto-auth (for Agent and Proxy)

[VioletHynes]

This, together with the plugin changes included in the go.mod, should enable Vault Agent and Proxy to use Azure Workload Identity Federation for authentication.

I used the following for a successful WIF login:

    auto_auth {
      method {
        type      = "azure"
        config = {
          authenticate_from_environment = true
          role = "dev-role"
          resource = "https://management.azure.com/"
        }
      }
    }

Tested in our legacy Azure tenant with Vault and Agent running in K8S, with WIF enabled, e.g.:

kubectl apply -f - <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: vault-agent
  labels:
    azure.workload.identity/use: "true"
spec:
  serviceAccountName: vault-agent-service-account
  containers:
  - name: vault-agent
    image:  violethyneshashicorp/vault:aks14
    args:
    - agent
    - -config=/etc/config/agent-config.hcl
    volumeMounts:
    - name: config-volume
      mountPath: /etc/config
  volumes:
    - name: config-volume
      configMap:
        name: agent-config
EOF

I couldn't see a good way to test these changes, as it essentially entirely relies on Azure APIs. We don't have any right now, either, likely for similar reasons. I'm open to suggestions, but I worry it might get mock-crazy.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[peteski22]

LGTM added a few suggestions 😄

[peteski22]

Suggested change

	// getTokenFromIdentityEndpoint is kept for backwards compatibility purposes. Using the
	// newer APIs and the Azure SDK should be preferred over this mechanism.
	// getTokenFromIdentityEndpoint is kept for backwards compatibility purposes. Using the
	// newer APIs and the Azure SDK should be preferred over this mechanism.
	// Deprecated: use getAzureTokenFromEnvironment.

Should we mark this with deprecated to give folks a warning in their IDE?

[VioletHynes]

I don't know that it's deprecated, per se - it's the 'old'/'legacy' way of doing things but I don't think it's deprecated from a Vault-product standpoint or an Azure product standpoint.

[github-actions]

Build Results:
All builds succeeded! ✅