Merge bitcoindevkit/bdk-cli#128: Check if psbt is final, before broadcasting

110be3fcfe0a562038791c7faf30a6684db6bc87 check if psbt is final, before broadcasting (rajarshimaitra)

Pull request description:

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  ### Description

  Fixes #97

  Add a finalizer check before broadcasting psbts.

  ### Checklists

  #### All Submissions:

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

ACKs for top commit:
  notmandatory:
    tACK 110be3fcfe0a562038791c7faf30a6684db6bc87

Tree-SHA512: 898908062bbfbe90ed296afbd83179066bfc2cb42538b3ff1c91a6da6c174ec4dbb0cd013dd766ec12ea944b7f0673ad36552aa0a423d2afbc94edcf43084d5c