Merge bitcoindevkit/bdk#1306: Implement `ElectrumExt` for all that implements `Electr...

ce8c617c9d562a54728c6cafc0562a66bae56b3e feat(electrum): impl `ElectrumExt` for all that impl `ElectrumApi` (志宇)

Pull request description:

  ### Description

  Implement `ElectrumExt` for all that implements `ElectrumApi`.

  I realized this while looking into #1171.

  https://github.com/bitcoindevkit/bdk/blob/2196685cca741a249f4a5b53c7609162469714ba/crates/electrum/tests/test_electrum.rs#L54-L55

  Line 55 here should not be necessary.

  ### Changelog notice

  * Changed to implement `ElectrumExt` for all that implements `ElectrumApi`.

  ### 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

ACKs for top commit:
  notmandatory:
    ACK ce8c617c9d562a54728c6cafc0562a66bae56b3e

Tree-SHA512: d95193a9a356a0aa3f467db3b8c7bf2cfdf83afd520f5254721912cfe82355de1198256b90323e51ffdb0bf955ff923a9ceb8b4541d4a5ad2331ff0bf26ab937