Merge bitcoindevkit/bdk#1950: feat!(chain): implement `first_seen` tracking

Merge bitcoindevkit/bdk#1950: feat!(chain): implement `first_seen` tracking

8b17bcfc03c39cd35415742bc31293b7969d1024 feat!(chain): implement `first_seen` tracking (uvuvuwu)

Pull request description:

  ### Description

  This PR solves issue #1947 by implementing `first_seen` tracking.

  * Added `first_seen` field to `TxGraph` and `ChangeSet` so that `first_seen` timestamp can be added when inserting a new seen-at using `insert_seen_at`.

  * `first_seen` added to `TxNode` as a way to retrieve the first-seen timestamp for a transaction.

  * `first_seen` added to `ChainPosition::Unconfirmed` to order unconfirmed transactions by `first_seen`.

  * New tests have been added for the above described functionalities.

  ### Changelog notice

  * Add tracking first-seen timestamps of transactions

  <!-- Notice the release manager should include in the release tag message changelog -->
  <!-- See https://keepachangelog.com/en/1.0.0/ for examples -->

  ### 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

ACKs for top commit:
  evanlinjin:
    ACK 8b17bcfc03c39cd35415742bc31293b7969d1024

Tree-SHA512: effb95704181f80ca25b63f5cf42b7a9357895647abefa91d4cc7ade7604bcfa4cca663d3916b935b520d935cf562412c5238f96636a428d115bf56b1957c9d5