Add ancestor package computation to CanonicalView

When building a transaction that spends UTXOs from unconfirmed parents, the wallet needs to know how much extra fee the child must pay to bring the ancestor package to a target feerate. This requires walking the unconfirmed ancestor graph, accumulating total weight and fees, and computing the deficit against the target. Related: [bitcoindevkit/bdk_wallet#48](https://github.com/bitcoindevkit/bdk_wallet/issues/48)