Restrict `drain_to` usage

Restrict `drain_to` usage

Before this commit, you could create a transaction with `drain_to` set
without specifying recipients, nor `drain_wallet`, nor `utxos`. What would
happen is that BDK would pick one input from the wallet and send
that one to `drain_to`, which is quite weird.
This PR restricts the usage of `drain_to`: if you want to use it as a
change output, you need to set recipients as well. If you want to send
a specific utxo to the `drain_to` address, you specify it through
`add_utxos`. If you want to drain the whole wallet, you set
`drain_wallet`. In any other case, if `drain_to` is set, we return a
`NoRecipients` error.

Fixes #620