Merge bitcoindevkit/bdk#1756: fix(electrum): prevent `fetch_prev_txout` from querying...

d4ef26611b83baefac65fffc0758c6ac55ade9cb test(electrum): `fetch_prev_txout` does not process coinbase transactions (Wei Chen)
0944b352156fcd797a5c199a462ccaed3761db0a fix(electrum): prevent `fetch_prev_txout` from querying coinbase transactions (Wei Chen)

Pull request description:

  Fixes #1698.

  ### Description

  `fetch_prev_txout` should not try to query the prevouts of coinbase transactions, as it may result in the Electrum server returning an error which would cause the overall `sync`/`full_scan` to fail. Without this critical bug fix,  `bdk_electrum` will crash when someone tracks an address being mined to.

  ### Notes to the reviewers

  ### Changelog notice

  * `fetch_prev_txout` no longer queries coinbase transactions.

  ### 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

  #### Bugfixes:

  * [ ] 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:
  evanlinjin:
    ACK d4ef26611b83baefac65fffc0758c6ac55ade9cb

Tree-SHA512: 6423f9486e84f204cf756117cabff35ce79ee169efa43a059c1669ad0f7193b58299eee7c5672af35ab070ed8011637b0a1904866ce2f2fa4580ddc3f9f2d982