Added contracts for the Cashier Tinkoff Driver

Author: Andrey Helldar

src/Cashier/Authentication/Auth.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Helldar\Contracts\Cashier\Authentication;
+
+interface Auth
+{
+    public function accessToken(Client $client): Credentials;
+}

src/Cashier/Authentication/Client.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Helldar\Contracts\Cashier\Authentication;
+
+interface Client
+{
+    public function clientId(string $client_id): self;
+
+    public function getClientId(): string;
+
+    public function clientSecret(string $client_secret): self;
+
+    public function getClientSecret(): string;
+}

src/Cashier/Authentication/Credentials.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Helldar\Contracts\Cashier\Authentication;
+
+use DateTimeInterface;
+use Helldar\Contracts\Support\Arrayable;
+
+interface Credentials extends Arrayable
+{
+    public function __construct(array $items = []);
+
+    public function getClientId(): string;
+
+    public function getAccessToken(): string;
+
+    public function getExpiresIn(): DateTimeInterface;
+}