Stop using singleton instance on Dart bindings

[erdemyerebasmaz]

This PR implements frb v2 Methods support.

• connect now returns a BindingsBreezServices which we can call BreezServices API from.

TODO:

• [ ] Add migration guide.
• [ ] See if it's possible to get rid of BreezSDK wrapper class as a whole, or slim it down as much as possible
  • For example, checking if Dart streams can be managed on Rust side via Rust call Dart or
  • Generating event listener logic via: Extra Dart code