Merge bitcoindevkit/bdk#805: electrum: add validate_domain to ElectrumBlockchainConfig

2451c00268a8b518212c59132c263bc186d1b0f9 electrum: add validate_domain to ElectrumBlockchainConfig (Igor Cota)

Pull request description:

  ### Description

  The purpose of the PR is to be able to configure both `stop_gap` **and** `validate_domain`. Perhaps there are nicer ways.

  #### 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:

  * [x] This pull request breaks the existing API
  * [x] I'm linking the issue being fixed by this PR

  Issue in https://github.com/bitcoindevkit/bdk/issues/804

ACKs for top commit:
  notmandatory:
    ACK 2451c00268a8b518212c59132c263bc186d1b0f9

Tree-SHA512: e10e3a027c202c8e680e5181f2a98c9ca30fa7773660cc47bef93f557cdc94c09e46b154b0edee65e498a49bc997cc9172eb3782dc4310d5775c0763d8c2ca4a