Merge bitcoindevkit/bdk#625: Restrict `drain_to` usage

6a150368674046f796f5c37755896f16d8345fbc Restrict `drain_to` usage (Daniela Brozzoni)

Pull request description:

  ### Description
  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

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

  * [x] This pull request breaks the existing API - kinda?
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    ACK 6a150368674046f796f5c37755896f16d8345fbc

Tree-SHA512: 69076977df37fcaac92dd99d2f2c9c37098971817d5b0629fc7e3069390eb5789331199b3b7c5d0569d70473f4f37e683a5a0b30e2c6b4e2ec22a5ef1d0f2d77

diff --cc src/wallet/mod.rs
Simple merge

diff --cc src/wallet/tx_builder.rs
Simple merge