Added `add_foreign_utxo`

Added `add_foreign_utxo`

To allow adding UTXOs external to the current wallet.
The caller must provide the psbt::Input so we can create a coherent PSBT
at the end and so this is compatible with existing PSBT workflows.

Main changes:

- There are now two types of UTXOs, local and foreign reflected in a
`Utxo` enum.
- `WeightedUtxo` now captures floating `(Utxo, usize)` tuples
- `CoinSelectionResult` now has methods on it for distinguishing between
local amount included vs total.