Add Dataset.from_spark
Adds static method Dataset.from_spark to create datasets from Spark DataFrames.

This approach alleviates users of the need to materialize their dataframe---a common use case is that the user loads their dataset into a dataframe, uses Spark to apply some transformation to some of the columns, and then wants to train on the dataset.

Related issue: https://github.com/huggingface/datasets/issues/5678