Add `allow_shrinking` to a non-signed, non-broadcasted transaction

[terror]

We want to be able to use TxBuilder::allow_shrinking() on a transaction that is created using Wallet::build_tx() but currently TxBuilder::allow_shrinking() is only allowed on transactions built with Wallet::build_fee_bump(txid), which requires a previous transaction to be signed and broadcasted to the network. Is there a way to use TxBuilder::allow_shrinking() without signing or broadcasting a transaction?