feat(esplora): always fetch prevouts

feat(esplora): always fetch prevouts

Prevouts are needed to calculate fees for transactions. They are
introduced as floating txouts in the update `TxGraph`. A helper method
`insert_prevouts` is added to insert the floating txouts using the
`Vin`s returned from Esplora.

Also replaced `anchor_from_status` with `insert_anchor_from_status` as
we always insert the anchor into the update `TxGraph` after getting it.

Also removed `bitcoin` dependency as `bdk_chain` already depends on
`bitcoin` (and it's re-exported).