Per discussion in our last backlog review call, since the bdk wallet APIs are opinionated about how to correctly build and use an on-chain wallet we should make unique internal (change address) and external (receive address) descriptors mandatory when creating a new Wallet. This will also allow us to remove current behavior of getting new change addresses from the external descriptor if no internal descriptor is given. Using external addresses for internal (change) breaks other wallet features like asking the tx builder to create transactions that don't spend change.

If a user wants to create a wallet without an internal descriptor they can do what ever they want using the lower level bdk_chain and other crates.