We also need to add new methods into UserService for the UserApi interface. Let's add new methods into the service, as follows:

findUserByUsername(): Finds and returns a user based on a given username.

createUser(): Adds a new signed-up user to the database.

getSignedInUser(): Creates a new model instance of SignedInUser that holds the refresh token, access token (JWT), user ID, and username.

getAcceessToken(): Generates and returns a new access token (JWT) for a given valid refresh token.

removeRefreshToken(): Removes the refresh token from the database. It is called when the user wants to sign out.