Merge bitcoindevkit/bdk#1041: Add `bitcoind_rpc` chain source module.

85c62532a55cfc94eade4d20ca3075dc4cd4882e docs(bitcoind_rpc): better `Emitter::mempool` explanation (志宇)
b69c13ddf6aa7cfb9be8c841b255e7f5f13ad328 example_bitcoind_rpc: tweaks (志宇)
5f34df8489fedae2aa3fd001036cc9ef6abe9a7a bitcoind_rpc!: bring back `CheckPoint`s to `Emitter` (志宇)
57590e0a1f2dad09a63fadb11f01e9f704cdcffb bitcoind_rpc: rm `BlockHash` from `Emitter::last_mempool_tip` (志宇)
6d4b33ef91a6c3e3443f6321cf3e3d186f77c595 chain: split `IndexedTxGraph::insert_tx` into 3 methods (志宇)
4f5695d43add3eab37ab12e897ac7c49f0d0787e chain: improvements to `IndexedTxGraph` and `TxGraph` APIs (志宇)
150d6f8ab6cd1eb1c9448d61e7bd71db0dd32a01 feat(example_bitcoind_rpc_polling): add example for RPC polling (志宇)
4f10463d9eaad9365b87dd99d49f0ddb8be673ee test(bitcoind_rpc): add no_agreement_point test (志宇)
a73dac2d91b29c4ba05f606f81e511fbf1f9ec7f test(bitcoind_rpc): initial tests for `Emitter` (志宇)
bb7424d11d3a4bd837ddde0f42f9abd93d56aee1 feat(bitcoind_rpc): introduce `bitcoind_rpc` crate (志宇)
240657b1674ad901c77090ee1fa96dc0d71e91d7 chain: add batch-insert methods for `IndexedTxGraph` (志宇)
43bc813c6498b5b021d70a6127ff7e57d7337813 chain: add helper methods on `CheckPoint` (志宇)
b3db5ca9df4302fddcd5474569849371f2be7e2d feat(chain): add `AnchorFromBlockPosition` trait (志宇)
f795a43cc72fdb4ef26ca349c4cb4f4bfd3b90b5 feat(example_cli): allow chain specific args in examples (志宇)

Pull request description:

  ### Description

  This PR builds on top of #1034 and adds the `bitcoind_rpc` chain-src module and example.

  ### Notes to the reviewers

  Don't merge this until #1034 is in!

  ### Changelog notice

  * Add `bitcoind_rpc` chain-source module.
  * Add `example_bitcoind_rpc` example module.
  * Add `AnchorFromBlockPosition` trait which are for anchors that can be constructed from a given block, height and position in block.
  * Add helper methods to `IndexedTxGraph` and `TxGraph` for batch operations and applying blocks directly.
  * Add helper methods to `CheckPoint` for easier construction from a block `Header`.

  ### Checklists

  * [x] Add test: we should detect when an initially-confirmed transaction is "unconfirmed" during a reorg.
  * [x] Improve `example_bitcoind_rpc`: add `live` command.
  * [x] Improve docs.
  * [x] Reintroduce `CheckPoint`.

  #### 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
  * [x] I've added docs for the new feature

ACKs for top commit:
  notmandatory:
    Re ACK 85c62532a55cfc94eade4d20ca3075dc4cd4882e

Tree-SHA512: 88dbafbebaf227b18c69f2ea884e3e586bf9c11e5e450eb4872ade1d1ccd5cf1e33ce9930a6f5aa918baa3e92add7503858b039b8c9d553a281ad6d833f08a49