Merge bitcoindevkit/bdk#1616: feat(wallet)!: enable RBF by default on TxBuilder

f15551b56fb4eb91bc0be5f940950c40ac465084 feat(wallet)!: enable RBF by default on TxBuilder (Luis Schwab)

Pull request description:

  ### Description

  Closes #791

  This PR enables RBF by default on `TxBuilder`

  ### Notes to the reviewers

  <!-- In this section you can include notes directed to the reviewers, like explaining why some parts
  of the PR were done in a specific way -->

  ### Changelog notice
  - On TxParams, `rbf` becomes `sequence`
  - Added `set_exact_sequence()`, so the user can define an arbitrary sequence value
  - `n_sequence` now defaults to `0xFFFFFFFD`
  - Adjusted tests accordingly

  ### 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:

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

  #### Bugfixes:

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

ACKs for top commit:
  notmandatory:
    ACK f15551b56fb4eb91bc0be5f940950c40ac465084

Tree-SHA512: 784538ffd102f315a7a96bd7f9afcdca7d35252ee2b4489d9853797c7f6d8cf95537f0dea3855ea9fffc9bf148c81a8c090cfdfd1e94144b9343651129ab9504