Add add_data enables op_return for bdk-cli

[waterst0ne]

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.

Notes to the reviewers

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [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

[notmandatory]

Overall looks good! I made a few comments above. Also you need to run cargo clippy and fix what ever clippy tells you to.

[waterst0ne]

Overall looks good! I made a few comments above. Also you need to run cargo clippy and fix what ever clippy tells you to.

All changes have been updated, everything looks good to go! Thank you for testing @notmandatory . 🚀

[waterst0ne]

Rebased on the latest master ready for final review please.