Add CASEHold scenario

[yifanmai]

CaseHOLD (Case Holdings On Legal Decisions) is a multiple choice question answering scenario where the task is to identify the relevant holding of a cited case (Zheng et al, 2021).

[yifanmai]

Hi @ryokawajp and @mtake, please review this pull request.

By approving, you acknowledge that:

• You will be listed as a co-author of the merge commit
• You agree to license your work under Apache 2.0

I have made the following changes have been made on top of your original commit:

• Used the test split instead of the validation split (despite the comment, the test split does have gold labels)
• Removed the unneeded call to list_datasets()
• Removed writing of JSONL files
• Simplified the dataset loading logic
• Edited the description and metadata
• Changed all group and scenario names to casehold for consistency - removed QA because existing references in the literature call this CaseHOLD rather than CaseHOLDQA

[ryokawajp]

lgtm. Thanks.