Allow serialization of Update

[icota]

Description

The rationale behind this is to allow a separate instance of the same wallet be updated on a device that's not necessarily connected. Like a hardware wallet.

A connected device gets the response, serializes it with serde and passes the Update to the air-gapped one.

Depends on bitcoindevkit/bdk#1954

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[luisschwab]

Closing in favor of #231.

[evanlinjin]

@luisschwab #231 cannot directly replace this PR. Relying on changesets mean that order of updates matter, and for @icota's usecase, it seems they rather not depend on that!

This PR is conceptACK for me, but implementation NACK. The problem is with the serialization of CheckPoint (as mentioned here).