[TODO] Split `SpOut` in `SpMeta` and `TxOut`

[nymius]

Description

TxGraph already stores the TxOuts and its changeset has them referenced by OutPoint. Although is nice to have them together, keys are not stored together with TxOuts in the usual setup, so I don't think bdk-sp should do the contrary. SpMeta will be a new structure containing the optional label and the tweak and will be stored indexed by OutPoint.

Use case

Additional context