refactor: restructure balance

[reez]

Description

Refactor Balance to Use Dictionary and Struct Patterns

Notes to the reviewers

Previously in 0.30.0 Balance was a dictionary. Currently in the 1.0 migration Balance is an interface.

The main motivation for this PR came via testing our bdk-swift 1.0.0 bindings locally in an iOS app where Balance was an interface; it made me want Balance to go back to being a dictionary instead of an interface if possible.

Example: In bdk-swift Balance is more ergonomic as a concrete type in my opinion (a struct instead of a protocol); an example where that is nice is if Balance is a struct it can be instantiated and mocked out, as opposed to if Balance is a protocol then a MyBalance struct needs to be created that conforms to the protocol Balance.

Key Changes:

• In UDL, Balance is now a dictionary instead of an interface.
• In lib.rs, Balance is now a struct instead of an impl

Tests across all platforms have been updated to reflect this refactor.

Changelog notice

Checklists

All Submissions:

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

ACK b66524b

Makes sense for bindings to make this into a dictionary. Be aware that the "trusted" balances should be going away based on conversation in bitcoindevkit/bdk#797, but can do that in a different PR once the change is made to BDK.

[reez]

Rebased on master

[thunderbiscuit]

ACK 05ce7da.