Started methods implementation, added login and check auth state methods. Added project license.

src/ClientInterface.php

@@ -30,6 +30,8 @@ interface ClientInterface {
    * Check if user logged in.
    *
    * @return bool
+   *
+   * @throws \Psr\Http\Client\ClientExceptionInterface
    */
   public function isLoggedIn(): bool;
 
@@ -48,6 +50,8 @@ interface ClientInterface {
    *
    * @return TopicInterface
    *   Object with populated topic data.
+   *
+   * @throws \Psr\Http\Client\ClientExceptionInterface
    */
   public function getTopic(int $id): TopicInterface;