Merge bitcoindevkit/bdk#731: Implement Deref<Target=Client> for RpcBlockchain

a8f9f6c43adb90c9ceced21da6b6bafaa90d7af9 RpcBlockchain derefs to the underlying RPC Client (rajarshimaitra)

Pull request description:

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  ### Description

  For the same reason as #705  and #722 ..

  ### 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:
  afilini:
    ACK a8f9f6c43adb90c9ceced21da6b6bafaa90d7af9

Tree-SHA512: 81e596fe451c275ca0ce27ee7ac9cf7e88433775603021c2dd1cd26a26558531cf74f81ef05d0ae9d5d0e59e91196e3ac6d38c0f4853b1889ddf822d8e63e178