ci: use Terraform for managing MailGun account

Fix #1302

[skip ci]

Author: php-coder

infra/terraform/.terraform.lock.hcl

@@ -60,6 +60,10 @@
   $ curl -sS -H 'Content-Type: application/x-www-form-urlencoded' -H 'Cache-Control: no-cache' -d "api_key=$UPTIMEROBOT_TOKEN" 'https://api.uptimerobot.com/v2/getPSPs' | jq '.psps[].id'
   1234
   ```
+* Import existing MailGun configuration (optionally; only the first time)
+  ```console
+  $ terraform import mailgun_domain.mystamps us:my-stamps.ru
+  ```
 * Plan and apply:
   ```console
   $ terraform plan -out terraform.tfplan

infra/terraform/README.md

@@ -1,5 +1,4 @@
 # @todo #1000 CI: validate and check Terraform configuration
-# @todo #1000 Terraform: add Mailgun
 
 # How to obtain a token: https://docs.digitalocean.com/reference/api/create-personal-access-token/
 variable "do_token" {
@@ -18,6 +17,27 @@ variable "uptimerobot_token" {
   sensitive   = true
 }
 
+# How to create API Key:
+# - https://app.mailgun.com/settings/api_security
+# - https://help.mailgun.com/hc/en-us/articles/203380100-Where-can-I-find-my-API-keys-and-SMTP-credentials
+variable "mailgun_token" {
+  description = "MailGun API key"
+  type        = string
+  sensitive   = true
+}
+
+# Mailgun provider docs: https://registry.terraform.io/providers/wgebis/mailgun/0.7.7/docs
+provider "mailgun" {
+  api_key = var.mailgun_token
+}
+
+# https://registry.terraform.io/providers/wgebis/mailgun/0.7.7/docs/resources/domain
+resource "mailgun_domain" "mystamps" {
+  name   = "my-stamps.ru"
+  region = "us"
+}
+
+
 # Digital Ocean provider docs: https://registry.terraform.io/providers/digitalocean/digitalocean/2.28.1/docs
 provider "digitalocean" {
   token = var.do_token

infra/terraform/my-stamps.tf

@@ -1,2 +1,3 @@
 do_token = "<insert-token-here>"
 uptimerobot_token = "<insert-token-here>"
+mailgun_token = "<insert-token-here>"

infra/terraform/terraform.tfvars.example

@@ -10,5 +10,9 @@ terraform {
       source  = "vexxhost/uptimerobot"
       version = "0.8.2"
     }
+    mailgun = {
+      source  = "wgebis/mailgun"
+      version = "0.7.7"
+    }
   }
 }