feat: implement mail-login directadmin plugin

exec/lib/DirectAdminContext.php

@@ -0,0 +1,56 @@
+<?php
+declare(strict_types=1);
+
+final class DirectAdminContext
+{
+    private function __construct(
+        private string $username,
+        private string $sourceHost,
+        private string $serverName,
+        private string $method
+    ) {
+    }
+
+    /**
+     * @param array<string,mixed> $server
+     */
+    public static function fromServer(array $server, Settings $settings): self
+    {
+        $username = trim((string)($server['USERNAME'] ?? $server['USER'] ?? ''));
+        if (!preg_match('/^[A-Za-z0-9][A-Za-z0-9._-]{0,31}$/', $username)) {
+            throw new InvalidArgumentException('Invalid DirectAdmin username');
+        }
+        $host = Settings::normalizeHost((string)($server['HTTP_HOST'] ?? $server['SERVER_NAME'] ?? ''));
+        if ($host === '') {
+            throw new InvalidArgumentException('Missing source host');
+        }
+        $serverName = $settings->sourceServerName();
+        if ($serverName === '') {
+            $serverName = $settings->sanitizeServerName(explode('.', $host)[0] ?? '');
+        }
+        if ($serverName === '') {
+            throw new InvalidArgumentException('Invalid source server name');
+        }
+        return new self($username, $host, $serverName, strtoupper((string)($server['REQUEST_METHOD'] ?? 'GET')));
+    }
+
+    public function username(): string
+    {
+        return $this->username;
+    }
+
+    public function sourceHost(): string
+    {
+        return $this->sourceHost;
+    }
+
+    public function serverName(): string
+    {
+        return $this->serverName;
+    }
+
+    public function method(): string
+    {
+        return $this->method;
+    }
+}