Merge bitcoindevkit/bdk#1865: chore(examples): make change descriptor optional on...

78ca4b0319740102b2989585c1f53640f1934e53 chore: make change descriptor optional on example_wallet_rpc (Luis Schwab)

Pull request description:

  ### Description

  This PR makes passing a `CHANGE_DESCRIPTOR` optional on `example_wallet_rpc`, as per #1533.

  ### Notes to the reviewers

  Before instantiating a `Wallet` it checks if `args.change_descriptor` is Some. If true calls `Wallet::create`, else calls `Wallet::create_single`.

  #### 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

ACKs for top commit:
  ValuedMammal:
    ACK 78ca4b0319740102b2989585c1f53640f1934e53

Tree-SHA512: ddca4b2a9931df1a067fbe86adc43c5f62f120cf76c9a1099995fd8a063303fe8e47e7b15e186c40fc347c3ffa350a45f92e129e724e2d99e1e3732d76faa2ee