Merge bitcoindevkit/bdk#1423: fix(persist): add default feature to enable bdk_chain/std

Merge bitcoindevkit/bdk#1423: fix(persist): add default feature to enable bdk_chain/std

a577c22b12f4edf0b2f2f266fb13656caf1d21c4 fix(persist): add default feature to enable bdk_chain/std (Steve Myers)

Pull request description:

  ### Description

  This PR adds a `default` feature to `bdk_persist` so it can be build on its own.  Once #1422 is done we can remove the `default`again.

  ### Notes to the reviewers

  I need to be able to build `bdk_persist` on its own so I can publish it to crates.io.

  ### 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:
  ValuedMammal:
    ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4
  oleonardolima:
    ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4
  storopoli:
    ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4

Tree-SHA512: 8b07a9e4974dec8812ca19ce7226dcaece064270a0be8b83d3c326fdf1e89b051eb0bd8aa0eda9362b2c8233ecd6003b70c92ee046603973d8d40611418c3841