Merge bitcoindevkit/bdk#2263: feat(chain): add ChainPosition::confirmations_lower_bound

7ebc9d3ca216eafb11fd1d976955f9e3500108c6 feat(chain): add ChainPosition::confirmations_lower_bound (Dmenec)

Pull request description:

  Suggested by @nymius in https://github.com/bitcoindevkit/bdk/pull/2246#discussion_r3796478212.

  ### Description

  Adds `ChainPosition::confirmations_lower_bound(tip)`, which returns the number of blocks mined on top of a confirmed position's block, given the chain `tip`. Returns `None` if the position is unconfirmed, or if the confirmation height is above `tip`.

  ### Changelog notice

  - Added `ChainPosition::confirmations_lower_bound`, returning the number of blocks mined on top of a confirmed position's block given the chain tip.

  ### 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 `just p` before committing

  #### New Features:

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

ACKs for top commit:
  evanlinjin:
    ACK 7ebc9d3ca216eafb11fd1d976955f9e3500108c6

Tree-SHA512: 32bc2ff794f9aa7a502589b6d8feca215c92dda1075b0dae25c7d5bb48d9c62f2af0c138bf47367bc75e5c0ee176e7290e0eeea152681b2dc172375cba6ebb37