FEAT Metadata for datasets should allow fields as string OR list of strings

[romanlutz]

Currently, we accept only string. This issue is to investigate whether we can allow string OR a list of authors as detailed in the post below:   Originally posted by @rdheekonda in https://github.com/Azure/PyRIT/pull/210#discussion_r1669316779

Same applies for other fields: https://github.com/Azure/PyRIT/pull/210#discussion_r1669316935