Merge bitcoindevkit/bdk#537: refactor wallet address caching into its own public...

Merge bitcoindevkit/bdk#537: refactor wallet address caching into its own public method

edf2f0ce06d27a1434202b79306cd1e998b5614c refactor wallet address caching into its own public method for offline wallet use (a5an0)

Pull request description:

  ### Description

  Currently, the only way to ensure that a wallet's internal database has addresses loaded and cached is through `Wallet::sync`: that function generates and caches up to a number of addresses if they aren't already in the database, and then uses the wallet's blockchain client to sync those addresses. If you are using an offline wallet, there is no mechanism to ensure that the database has addresses loaded. This is a problem for usecases like an offline wallet being used as a multisig signer and wanting to validate change addresses as `Wallet::is_mine` will only work properly if the owned-address is loaded in the database.

  This PR takes the address caching functionality out of `Wallet::sync` and puts it in a new public method that is available to offline wallets. `Wallet::sync` uses this method internally.

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

Top commit has no ACKs.

Tree-SHA512: 8ddc58d71457163bb20ff663ac508feb4e77000688161b63841a94db30b3f29f60f35fa2467bd99546123148873e3aed11e2f13ae6cbceda6605b83c227d9079