Merge bitcoindevkit/bdk#744: Add psbt_signer.rs example

fa998de4b1849adcf43351dbec05b3de43e6694e Add psbt_signer.rs example (Steve Myers)

Pull request description:

  ### Description

  Adding a simple example of how to create a PSBT with a watch only wallet and then sign it with a signing wallet.

  ### Notes to the reviewers

  This example was inspired by a question from a user.

  ### Changelog notice

  none.

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

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

ACKs for top commit:
  danielabrozzoni:
    ACK fa998de4b1849adcf43351dbec05b3de43e6694e
  afilini:
    ACK fa998de4b1849adcf43351dbec05b3de43e6694e

Tree-SHA512: 2e178ee59ce94eb1e9b5616a499e106e1d2843589036bdb6cff2c987e280588ad9989b026cdbf01290cc60f02eccbc410a3e1d1cd45eb4b8ff04353ae31b23ea