Merge bitcoindevkit/bdk#722: Implement Deref<Target=UrlClient> for EsploraBlockchain

baf7eaace66edcae8bf3252a962b9417b704ba26 Implement Deref<Target=UrlClient> for EsploraBlockchain (Vladimir Fomene)

Pull request description:

  ### Description

  There is currently no way to access the client from the EsploraBlockchain. This makes it difficult for users to extend it's functionality. This PR exposes both the reqwest and ureq clients. This PR is related to PR #705.

  ### 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:
    tACK baf7eaace66edcae8bf3252a962b9417b704ba26

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