Broaden transaction data requirements

[sander]

Context: This enables the QES creation use case where transaction data contains parameters for advanced e-signature creation. In these cases, the transaction data do not get hashed in their JSON representation format using a single hash algorithm. Instead, the X.509 credential format specifies specific ways of processing, that still meet the broadened requirements.

Proposed changes:

• When transaction_data_hashes_alg is omitted, do not require a default value of sha-256. This enables a credential format to specify hash algorithms.
• Broaden transaction_data_hashes specification in VP token. This enables advanced e-signature (AdES) processing rather than straight hashing of a serialised JSON array.

Draft PR: #421