These directly copy the iOS APIs, to minimize the branching needed in plugins with shared implementation code, and to facilitate the long-term goal of merging the iOS and macOS plugin headers. This does mean replicating the unfortunately non-idiomatic behavior of having `valuePublishedByPlugin:` sometimes return `nil` and sometimes return `NSNull`, instead of distinguishing between `nil` cases (if that's actually even necessary here) via a more specific API. In isolation I would definitely not design the API with this behavior, but consistency with iOS is the more important factor.

(Eventually I think we'll need a sort of "v2" of iOS plugin APIs since there are a number of strange behaviors that we're currently stuck with, but migrating iOS and macOS together to a new set of APIs won't be any harder than doing just iOS, and in the short to medium term consistency will help the ecosystem more that trying to pre-create better APIs as macOS-only.)

Also fixes `FlutterEngineRegistrar` to have a weak pointer to the engine. This should really already have been the case since plugins can retain the registrar, creating a likely cycle; it's now a guaranteed cycle (and failed unit tests designed to find cycles) without that since the engine itself is now keeping references to them.

Fixes flutter/flutter#124721

[C  , Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style