Extend frenet_optimal_trajectory to support more scenarios

[Aglargil]

Hi there, First of all, thank you for the amazing PythonRobotics library! It's an incredibly valuable resource for learning robotics algorithms. I've been working on the frenet_optimal_trajectory implementation lately, and noticed that it currently only handles Velocity Keeping for High Speed Trajectories. I'm interested in extending it to support additional scenarios mentioned in the paper, such as:

• Low Speed Trajectories
• Following, Merging, and Stopping

Would you be open to accepting contributions to frenet_optimal_trajectory to handle these cases? I'd be happy to submit PR to implement them.