Merge bitcoindevkit/bdk#2195: fix(electrum): do not pick unindexed outputs for histor...

2e3d52e80fdc504c718426b9255a30fc24b7ceda fix(electrum): do not pick unindexed outputs for history lookup (Zoe Faltibà)

Pull request description:

  ### Description

  `BdkElectrumClient::populate_with_txids` queries each transaction's confirmation status by calling `script_get_history` on the script of one of its outputs. It currently picks the first output unconditionally. This breaks for transactions which first output is an `OP_RETURN`, because Electrum servers don't index `OP_RETURN` scripts and will return an empty history. This is a real-world scenario: protocols like RGB place an `OP_RETURN` commitment as the first output of every transaction.

  ### Notes to the reviewers

  The fix selects the first output which script is not `OP_RETURN` or a `OP_FALSE OP_RETURN`. If a transaction has only `OP_RETURN`/`OP_FALSE OP_RETURN` outputs, we fall back to the script of any input's previous output to query history. The only case still skipped is a coinbase with all unindexed outputs, since coinbases have no parent to fall back on.

  ### Changelog notice

  ```
  Fixed:
  - `BdkElectrumClient::sync` now correctly retrieves confirmation status for transactions which first output is an `OP_RETURN` or `OP_FALSE OP_RETURN`
  ```

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  oleonardolima:
    ACK 2e3d52e80fdc504c718426b9255a30fc24b7ceda
  evanlinjin:
    ACK 2e3d52e80fdc504c718426b9255a30fc24b7ceda

Tree-SHA512: d87a0580c0db4e86ee6fce84a82ac56ef70378b46405bb54ca33f1a2a643feba6fa769c0ba2a9e01c77cc6308c777ccc34f89d548e46a1bbf036e31d8c3ddcd0