Extend the transaction data hashes response structure

The transaction_data_hashes_alg can be separate per transaction_data entry, but in the response it is expected that all transaction data hashes use the same transaction_data_hashes_alg.

Wouldn't it make more sense to make the transaction_data_hashes an array of objects, where each entry has a hash and a hash_alg, so that separate entries can be hashed using different algs?

This could also prevent conflicts where I ask for two transaction data hashes for the same credential id, one that only allows sha-256 and the other that only allows sha-512.

I think it could also help with #442, where the object could contain a transaction_data_index to point to the index from the request to make it easier to link between the request entry and the response entry

Start by reviewing the current transaction data hashes response structure described in this issue and compare it with the per-entry transaction_data requirements. Read issue #442 for the related linking concern. Done means the proposed response changes are specified consistently, including how separate algorithms and request-to-response entries are represented.