Merge bitcoindevkit/bdk#705: Implement `Deref<Target=Client>` for `ElectrumBlockchain`

c5952dd09a61b1cd2185c273e0b8bcb3fd6ed2dd Implement `Deref<Target=Client>` for `ElectrumBlockchain` (Alekos Filini)

Pull request description:

  ### Description

  As pointed out in https://github.com/bitcoindevkit/rust-electrum-client/pull/58#issuecomment-1207890096 there was no way to keep using the client once it was given to BDK.

  ### 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:
  rajarshimaitra:
    ACK c5952dd09a61b1cd2185c273e0b8bcb3fd6ed2dd

Tree-SHA512: fbfbada51c9426266c8960da5508ee07b196808f0d670a09a51962bd6eda9ccf585e209f5b99b5ab78a3d17af774bdb3e33ef36ac4f4d1ce7f2c3398ae4f6d0c