Merge bitcoindevkit/bdk#1733: feat(chain,wallet)!: Transitive `ChainPosition`

29b374e383bc9317b55a2a58019e1948e170fabd feat(chain,wallet)!: Transitive `ChainPosition` (志宇)

Pull request description:

  ### Description

  Change `ChainPosition` to be able to represent transitive anchors and unconfirm-without-last-seen values.

  As mentioned in https://github.com/bitcoindevkit/bdk/pull/1670#issuecomment-2484535021, we want this merged first so that we have minimal changes to the API after 1670 is merged.

  ### Changelog notice

  * Change `ChainPosition` so that it is able to represent transitive anchors and unconfirmed-without-last-seen values.

  ### 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:

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

ACKs for top commit:
  ValuedMammal:
    ACK 29b374e383bc9317b55a2a58019e1948e170fabd

Tree-SHA512: 58f22f38201304611341835f22f2526254009077cde6dfcd1f6051aec906ddae78f45ebd7900c35c0fb5165ed10e48a45a55fca73395edcf9bec2fb1daa1acc6