Merge bitcoindevkit/bdk-cli#105: Add add_data enables op_return for bdk-cli

Merge bitcoindevkit/bdk-cli#105: Add add_data enables op_return for bdk-cli

86b557972de9833105fbddefbaf1082cab4499f0 Add add_data enables op_return for bdk-cli (Pedro Felix)

Pull request description:

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  ### Description
  This enables users to send an arbitrary string message through a txn-output. The maximum size of this string-literal can be length of 80-bytes.
  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  ### Notes to the reviewers

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  of the PR were done in a specific way -->

  ### 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

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

  #### 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:
  notmandatory:
    ACK 86b557972de9833105fbddefbaf1082cab4499f0
  rajarshimaitra:
    tACK 86b557972de9833105fbddefbaf1082cab4499f0

Tree-SHA512: 53376da1489a7479f95971e7ec6ab6d011652552a865e9d76e9e055b16a1780e7b8ea4ff7e36845252fdf968b80875a81dce54b6f826ca0ddf8cd84b167278c4