Merge bitcoindevkit/bdk#831: Don't default to use `async`/`await` on `wasm32`

32912eaa05fd3bb4fa1577ca413dcedf909544d9 Don't default to use `async`/`await` on `wasm32` (Elias Rohrer)

Pull request description:

  ### Description

  We don't automatically want to make the interface `async` based on the used architecture, but now require the user to explicitly set the `async-interface` feature.

  #### 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:
  notmandatory:
    ACK 32912eaa05fd3bb4fa1577ca413dcedf909544d9

Tree-SHA512: 41a8f3ab29508a359a8c0d96994e4fa97e52c15e8b8003c1988bcfe036cafa81d6210d446ed825672ce52aff684ebb328dc61b8ae3d25cda6f51a3cae838de58