add avatar_url to login response, achievement_info response, and leaderboard_info responses

[Jamiras]

Provides plumbing to support RetroAchievements/RAWeb#3017

rc_api_process_login_server_response, rc_api_process_fetch_achievement_info_server_response, and rc_api_process_fetch_leaderboard_info_server_response now return avatar_url fields beside the username field. These should be used instead of calling rc_api_init_fetch_image_request with the username, as the avatar URL for a user who has changed their display name will not match and the default avatar image will be downloaded instead.

rc_client_user_get_image_url will automatically leverage the new avatar_url data. Clients using rc_api_init_fetch_image_request directly to fetch user images for any purpose should switch to use the avatar_url value instead.

If the server doesn't provide an avatar_url (for the brief period until the above PR is deployed), it will be constructed in the same manner as was previously handled by rc_api_init_fetch_image_request.