Merge bitcoindevkit/bdk#1998: refactor!: `Box` the changeset of `StoreErrorWithDump`

418753f01eef29b1266cc3aefa512f90060a9797 refactor!: `Box` changeset in `StoreErrorWithDump` (codingp110)

Pull request description:

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  ### Description
  Fixes #1991. As the linked issue mentions `Box`ing helps reduce memory overhead and [limits the size of enums with `StoreErrorWithDump` as variant](https://github.com/bitcoindevkit/bdk_wallet/pull/277#discussion_r2196831262).

  Breaking: The enum `StoreErrorWithDump` and the functions `load`,`dump` and `load_or_create` have been changed.

  ### Changelog Notice
  ```
     Changed
     - `changeset` field of `StoreErrorWithDump` is now of type `Option<Box<C>>`
  ```

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  evanlinjin:
    ACK 418753f01eef29b1266cc3aefa512f90060a9797

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