Tx Builder metadata

[buffrr]

Describe the enhancement

The Tx Builder is uniquely positioned to know all the details about a transaction being made with it. Foreign utxos, any special ordering requested, any explicitly specified locktime and possibly even the coin selection algorithm specified.

It would be valuable for the builder to generate metadata that can be optionally applied to the wallet something like wallet.apply_tx_with_metadata, allowing it to track foreign UTXOs, preserve transaction details, and even include comments or notes.

Use case

As a consumer of bdk, it's unintuitive that:

1. I would need to manually insert any foreign utxos already added to the builder using insert_txout otherwise I wouldn't be able to calculate/bump the fee for that transaction.
2. Wallet assumes double spends of those foreign utxos are not relevant during sync leaving them stuck as unconfirmed.
3. Transaction properties (ordering, explicitly specified locktime, coin selection) are not preserved during fee bumping

Also storing comments or additional metadata directly in the wallet database improves usability by eliminating the need to track them separately.