feat: account UTxOs REST handler

[whankinsiv]

This PR implements the /accounts/{stake_address}/utxos endpoint as part of #256 as well as #151.

Main changes:

• Added GetUTXOHashes query handler to chain_store, enabling the lookup of both block and transaction hashes for a set of UTxOIdentifiers.
• Added GetUTxOs query handler to utxo_state, enabling retrieval of full UTXOValue entries for a set of UTxOIdentifiers.
• Added and registered the /accounts/{stake_address}/utxos endpoint via handle_account_utxos_blockfrost.
• Added placeholder endpoint registrations and handlers for /accounts/{stake_address}/addresses/assets and /accounts/{stake_address}/addresses/total.

Endpoint flow:

1. Query account Shelley addresses from historical_accounts_state.
2. Query UTxO identifiers for those addresses from address_state.
3. Query transaction and block hashes from chain_store using UTxO identifiers.
4. Query UTxO value entries from utxo_state.
5. Construct REST response.

Next steps:

• Implement /accounts/{stake_address}/addresses/assets, following a similar pattern to retrieve asset balances via historical_accounts_state -> address_state -> utxo_state.