Add nested optuna sampling

[robogast]

• Nested sampling is supported in optuna: https://optuna.readthedocs.io/en/stable/tutorial/10_key_features/002_configurations.html
• Nested sampling is not supported in hydra
• Ideally we would like categorical choices to be config options, i.e. set choices in the default list
  • Problem: choices from the default list are actually a dictionary, not a list as required by categorical distribution
  • Problem: dictionaries are not always supported by storage backends https://optuna.readthedocs.io/en/stable/reference/generated/optuna.distributions.CategoricalDistribution.html#optuna.distributions.CategoricalDistribution
    • Mitigation: If the choice is a dictionary, replace it by a string (ex: f'key_option_{x}'), save the dictionary in some parallel dictionary (or as hidden key in the search space dictionary).