Get user profile endpoint.
MVP: Get user by API Key.
V2: Return user profile according to the OAuth2 token supplied. If user not exists - verify OAuth token, retrieve user profile from the provider and create Haiqu user instance.
Args: access_token (str): Token from OAuth provider, Google, IBM, etc. api_access_key (str): Haiqu API key, if user got an invitation with HAIQU_API_KEY.
Returns: UserModel: User profile object.
Successful Response