Merge bitcoindevkit/bdk#1351: fix: define and document `stop_gap`

7c1861aab9a76f2ba4a2ae372a16c9669a33f5de fix: define and document `stop_gap` (Jose Storopoli)

Pull request description:

  ### Description

  - changes the code implementation to "the maximum number of consecutive unused addresses" in esplora async and blocking extensions.
  - for all purposes treats `stop_gap = 0` as `stop_gap = 1`.
  - renames `past_gap_limit` to `gap_limit_reached` to indicate we want to break once the gap limit is reached and not go further in `full_scan`, as suggested in https://github.com/bitcoindevkit/bdk/issues/1227#issuecomment-1859040463
  - change the tests according to the new implementation.
  - add notes on what `stop_gap` means and links to convergent definition in other Bitcoin-related software.

  Closes #1227.

  ### Notes to the reviewers

  We can iterate over the wording and presentation of the `stop_gap` definition
  and details.

  ### Changelog notice

  - BREAKING: change `stop_gap` definition and effects in `full_scan`
    to reflect the common definitions in most Bitcoin-related software.

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

  #### Bugfixes:

  * [x] This pull request breaks the existing API
  * [x] 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 7c1861aab9a76f2ba4a2ae372a16c9669a33f5de

Tree-SHA512: d9bc5f8ebace47fa33f023ceddb3df5629ad3e7fa130a407311a0303ac59810e6527254efb9075f9e87bf37bec8655c4726eb9cb99f6b038fbeb742f79e995c0