Fix custom terms

[JR-1991]

This PR extends the library to support custom terms, which are treated differently than the standard license field. This enhancement addresses the use case raised in #46 regarding custom terms of use. Additionally, it resolves issue #45, where fields like SocialScienceNotes (which is a sinfle-valued compound) were parsed incorrectly, resulting in a DottedDict holding the raw response and triggering a ValidationError.

Implementation Details

Dataverse responses may include a set of custom term fields that appear at the root level of the JSON. This PR introduces a new CustomLicense class to encapsulate and parse these fields. The decision to extract this into a separate class is motivated by two key considerations:

• Both License and CustomLicense are exposed via the same license property on the Dataset, maintaining a clean API and avoiding the need to manage additional initialization properties.
• Mapping both to the same license field reflects Dataverse behavior, where either a standard license or custom terms may be present — but not both.

Example Usage

# Setting a custom license
dataset = dataverse.create_dataset()
dataset.license = CustomLicense(
    termsOfUse="This is a custom terms of use",
    confidentialityDeclaration="This is a custom confidentiality declaration",
    specialPermissions="This is a custom special permissions",
    restrictions="This is a custom restrictions",
    citationRequirements="This is a custom citation requirements",
    depositorRequirements="This is a custom depositor requirements",
    conditions="This is a custom conditions",
    disclaimer="This is a custom disclaimer",
)

• Closes custom terms can't be loaded #46
• Closes ValidationError compound fields in metadatablock social_science #45