feat: add native asset decoding to tx_unpacker and store balances in utxo_state

[whankinsiv]

This PR introduces native asset decoding support in tx_unpacker and integrate it with utxo_state storage. TxOutputs now carry full Values (lovelace + assets) instead of only lovelace. These are propagated through UTXODeltasMessage into ledger storage.

Main changes:

• Added Value and ValueDelta types to represent balances:

pub struct Value {
    pub lovelace: u64,
    pub assets: NativeAssets,
}
pub struct ValueDelta {
    pub lovelace: i64,
    pub assets: Vec<(PolicyId, Vec<NativeAssetDelta>)>,
}
pub struct NativeAsset {
    pub name: AssetName,  
    pub amount: u64,
}
pub struct NativeAssetDelta {
    pub name: AssetName,  
    pub amount: i64,
}

• Added map_value() in tx_unpacker/src/map_parameters to decode Pallas MultiEraValue into Value.
• Updated TxOutput to use Valueinstead of a raw u64.
• Refactored /utxo_state/state.rs to process and persist native asset vectors.
• Refactored /utxo_state/rest.rs to expose asset balances in REST responses.

Memory usage benchmark (epoch 300):
main: 2.465 GB
this branch: 2.854 GB

[sandtreader]

Looks great generally - just concern about use of expect() and would like unit tests extended too.

[whankinsiv]

All feedback has been implemented. Merging so that I can continue progress on #148.