feat: support API Key authentication to UniFi controller (#22)

examples/provider/provider.tf

@@ -1,7 +1,8 @@
 provider "unifi" {
   username = var.username # optionally use UNIFI_USERNAME env var
   password = var.password # optionally use UNIFI_PASSWORD env var
-  api_url  = var.api_url  # optionally use UNIFI_API env var
+  api_key = var.api_key  # optionally use UNIFI_API_KEY env var
+  api_url = var.api_url  # optionally use UNIFI_API env var
 
   # you may need to allow insecure TLS communications unless you have configured
   # certificates for your controller

examples/provider/test.auto.tfvars

@@ -1,5 +1,6 @@
 username = "tfacctest"
 password = "tfacctest1234"
+# api_key = "tfacctest1234"
 
 # this assumes the default port for acc testing
 api_url  = "https://localhost:8443/api/"

examples/provider/variables.tf

@@ -4,6 +4,9 @@ variable "username" {
 variable "password" {
 }
 
+variable "api_key" {
+}
+
 variable "api_url" {
 }