Use interface to access API

Allows for better testability.

osu.Game/Online/API/IAPIProvider.cs

@@ -18,6 +18,19 @@ namespace osu.Game.Online.API
         /// </summary>
         bool IsLoggedIn { get; }
 
+        /// <summary>
+        /// The last username provided by the end-user.
+        /// May not be authenticated.
+        /// </summary>
+        string ProvidedUsername { get; }
+
+        /// <summary>
+        /// The URL endpoint for this API. Does not include a trailing slash.
+        /// </summary>
+        string Endpoint { get; }
+
+        APIState State { get; }
+
         /// <summary>
         /// Queue a new request.
         /// </summary>
@@ -29,5 +42,32 @@ namespace osu.Game.Online.API
         /// </summary>
         /// <param name="component">The component to register.</param>
         void Register(IOnlineComponent component);
+
+        /// <summary>
+        /// Unregisters a component to receive state changes.
+        /// </summary>
+        /// <param name="component">The component to unregister.</param>
+        void Unregister(IOnlineComponent component);
+
+        /// <summary>
+        /// Attempt to login using the provided credentials. This is a non-blocking operation.
+        /// </summary>
+        /// <param name="username">The user's username.</param>
+        /// <param name="password">The user's password.</param>
+        void Login(string username, string password);
+
+        /// <summary>
+        /// Log out the current user.
+        /// </summary>
+        void Logout();
+
+        /// <summary>
+        /// Create a new user account. This is a blocking operation.
+        /// </summary>
+        /// <param name="email">The email to create the account with.</param>
+        /// <param name="username">The username to create the account with.</param>
+        /// <param name="password">The password to create the account with.</param>
+        /// <returns>Any errors encoutnered during account creation.</returns>
+        RegistrationRequest.RegistrationRequestErrors CreateAccount(string email, string username, string password);
     }
 }