return assetCommitment in ListTransactions

Returning the commitment is useful for coinjoin situations, since surjectionproof requires a complete list of asset commitments up front for the inputs.

Start at the ListTransactions implementation and trace the transaction fields it returns to callers. Check how asset commitments are represented and how the result supports coinjoin inputs requiring a complete list for surjectionproof; done means ListTransactions returns assetCommitment in that context.

Written by the indexing model from the issue text.