Add support for dynamic search spaces in PreferentialGPSampler

[resnant]

Feature Request

The PreferentialGPSampler in the optuna-dashboard currently lacks support for dynamic search spaces, leading to constraints in its application to complex problems. When attempting to use it with dynamic search spaces, users encounter warnings and errors such as

• UserWarning: Dynamic search space detected. Falling back to RandomSampler.
• NotImplementedError: The search space has changed. Dynamic search space is not supported in PreferentialGPSampler.

To overcome this limitation and enhance the applicability of PreferentialGPSampler to a broader range of optimization problems, I kindly request the implementation of dynamic search space support.