Enable authentication steps for AzureMsiAuthentication for reactive usage.

Closes gh-665.

spring-vault-core/src/main/java/org/springframework/vault/authentication/AzureMsiAuthentication.java

@@ -52,7 +52,7 @@ import org.springframework.web.client.RestOperations;
  * "https://docs.microsoft.com/en-us/azure/virtual-machines/windows/instance-metadata-service"
  * >Azure Instance Metadata service</a>
  */
-public class AzureMsiAuthentication implements ClientAuthentication {
+public class AzureMsiAuthentication implements ClientAuthentication, AuthenticationStepsFactory {
 
 	private static final Log logger = LogFactory.getLog(AzureMsiAuthentication.class);
 
@@ -144,6 +144,11 @@ public class AzureMsiAuthentication implements ClientAuthentication {
 		return createTokenUsingAzureMsiCompute();
 	}
 
+	@Override
+	public AuthenticationSteps getAuthenticationSteps() {
+		return createAuthenticationSteps(this.options);
+	}
+
 	private VaultToken createTokenUsingAzureMsiCompute() {
 
 		Map<String, String> login = getAzureLogin(this.options.getRole(), getVmEnvironment(), getAccessToken());