Request note asset witnesses before transaction execution

[PhilippGackstatter]

#1848 introduced lazy loading for asset witnesses. Asset witnesses are also needed for mutating the input vault and they are currently requested on-demand. But the note assets are known before tx execution starts and so they can be requested as part of fetching transaction inputs. One idea is to change the data store as follows and allow fetching some asset witnesses in bulk:

fn get_transaction_inputs(
    &self,
    account_id: AccountId,
    ref_blocks: BTreeSet<BlockNumber>,
    asset_vault_keys: Vec<Word>,
) -> impl FutureMaybeSend<
    Result<(PartialAccount, Option<Word>, BlockHeader, PartialBlockchain, Vec<AssetWitness>), DataStoreError>,
>;

We would then request all witnesses for assets in input notes.

[bobbinth]

Maybe it would make sense to get rid of our current TransactionInputs and replace the (PartialAccount, Option<Word>, BlockHeader, PartialBlockchain, Vec<AssetWitness>) tuple with new TransactionInputs which would look something like:

pub struct TransactionInputs {
    account: PartialAccount,
    account_seed: Option<Word>,
    block_header: BlockHeader,
    block_chain: PartialBlockchain,
    asset_witnesses: Vec<AssetWitness>,
}

[igamigo]

fn get_transaction_inputs(
    &self,
    account_id: AccountId,
    ref_blocks: BTreeSet<BlockNumber>,
    asset_vault_keys: Vec<Word>,
) -> impl FutureMaybeSend<
    Result<(PartialAccount, Option<Word>, BlockHeader, PartialBlockchain, Vec<AssetWitness>), DataStoreError>,
>;

Would it make more sense to make asset_vault_keys a BTreeSet<Word> here?

[bobbinth]

Another thing to consider (related to #1856) is that we may want to load the witness for the fee asset at the start of the transaction, because it will very likely be needed in the transaction epilogue.