Adjust return types for asset and tx endpoints

[the-Minister-0001]

This pull requests changes the return types for asset and tx endpoints in order to reflect the actual data returned by the blockfrost API. Additionally, Order is exposed to allow customization of pagination order.

Changed types:

• assets_by_id from AssetsInner to Asset to reflect GET /assets/{asset}
• transactions_by_hash from AddressTransactionsContentInner to TxContent to reflect GET /txs/{hash}
• transactions_utxos from TxContent to TxContentUtxo to reflect GET /txs/{hash}/utxos

This also addresses Issue #45

[vladimirvolek]

You are right! Thank you 💕