Merge bitcoindevkit/bdk#634: Get block hash by its height

2af678aa846a82657d13bcda2ac0b4a9db4e2b80 Get block hash by its height (Vladimir Fomene)

Pull request description:

  ### Description
  This PR create a new trait `blockchain::GetBlockHash` with a `get_block_hash` method which returns a block hash given the block height. This has been implemented for all blockchain backends.
  Fixes #603

  ### Notes to the reviewers

  I haven't updated the `CHANGELOG.md` and docs. Am I suppose to update it for this change?

  ### 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

  #### New Features:

  * [x] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [ ] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    ACK 2af678aa846a82657d13bcda2ac0b4a9db4e2b80

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