Add example for azure function app using key vault reference (#114)

add examples for function app using key vault reference

quickstart/201-function-app-key-vault-ref/providers.tf

@@ -0,0 +1,18 @@
+terraform {
+  required_version = ">=1.0"
+
+  required_providers {
+    azurerm = {
+      source  = "hashicorp/azurerm"
+      version = "~>3.8"
+    }
+  }
+}
+provider "azurerm" {
+  features {
+    key_vault {
+      purge_soft_delete_on_destroy    = true
+      recover_soft_deleted_key_vaults = true
+    }
+  }
+}