Add H2H authentication service and new example flows

- Implement H2H authentication service for secure client authentication
- Add new examples for authorization code flow and H2H auth service
- Include example for generating client secret
- Update core connector and token request functionality
- Enhance README with new features

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

src/Services/H2HAuthService.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Apoloz\AlfaBank\Services;
+
+use Apoloz\AlfaBank\Core\AlfaBankConnector;
+use Apoloz\AlfaBank\Data\Auth\TokenData;
+use Apoloz\AlfaBank\Enums\Environment;
+
+class H2HAuthService
+{
+    public function __construct(
+        protected AlfaBankConnector $connector,
+        protected string $clientId,
+        protected string $redirectUri,
+        protected Environment $environment = Environment::SANDBOX
+    ) {}
+
+    /**
+     * Step 1: Generate the URL for the user to authorize via Alfa ID.
+     *
+     * @param array<string> $scopes List of scopes (e.g. ['beneficiaries', 'payment'])
+     * @param string|null $state Optional state parameter for CSRF protection
+     */
+    public function getAuthUrl(array $scopes = ['beneficiaries'], ?string $state = null): string
+    {
+        $baseUrl = $this->environment === Environment::PRODUCTION
+            ? 'https://id.alfabank.ru/oidc/authorize'
+            : 'https://id-sandbox.alfabank.ru/oidc/authorize';
+
+        $params = [
+            'response_type' => 'code',
+            'client_id' => $this->clientId,
+            'redirect_uri' => $this->redirectUri,
+            'scope' => implode(' ', $scopes),
+        ];
+
+        if ($state) {
+            $params['state'] = $state;
+        }
+
+        return $baseUrl . '?' . http_build_query($params);
+    }
+
+    /**
+     * Step 2: Exchange the received code for Access & Refresh Tokens.
+     */
+    public function exchangeCode(string $code): TokenData
+    {
+        return $this->connector->authWithCode($code, $this->redirectUri);
+    }
+
+    /**
+     * Step 3: Refresh the token using a stored refresh token.
+     */
+    public function refreshToken(string $refreshToken): TokenData
+    {
+        return $this->connector->refreshToken($refreshToken);
+    }
+}