This commit adds a mempool acceptance check before broadcasting a given
transaction. To maintain the current behavior from
`BtcWallet.PublishTransaction`, the two errors, `ErrInMempool` and
`ErrAlreadyConfirmed` returned from `TestMempoolAccept` are ignored.
In this commit, we add a new interface, the `ScriptDesciptor` to
abstract over details of a given output script. The purpose of this
interface, and the taproot superset, is to be able to paper over the
differences of a p2wsh vs a p2tr output. With this new interface, we can
treat them as the same, but then use a type assertion to get at any
control block related methods if needed.
