Improve `txout` listing and balance APIs for redesigned structures

Improve `txout` listing and balance APIs for redesigned structures

Instead of relying on a `OwnedIndexer` trait to filter for relevant
txouts, we move the listing and balance methods from `IndexedTxGraph` to
`TxGraph` and add an additional input (list of relevant outpoints) to
these methods.

This provides a simpler implementation and a more flexible API.