Package-level declarations

According to "Transaction Data entries as defined in D3.1: UC Specification WP3" the encoding is JSON and every entry is serialized to a base64 encoded string

After D3.1: UC Specification WP3. However, this class is potentially a mistake in the draft spec vs test vector, currently we need it to be a sealed class with polymorphic serialization to get the structure method: {type: NAME} sealed class would instead serialize to method: NAME which might be the corrected implementation in the next draft.

Implements "Transaction Data entries as defined in D3.1: UC Specification WP3" leveraging upcoming changes to OpenID4VP