Makes InvokeMethod's reply a high-level response object, rather than
binary data, matching the abstraction level of the class (and the other
languages' implementations).
In support of that:
- Adds the logic to the codecs to decode response envelopes, which had
never been implemented.
- Adds a convience implementation of MethodResult that forwards to
lambdas, so that one-off invocation handlers are easier to write.
Also simplified BinaryMessenger's API so that subclasses only need to
implement one version of Send, rather than two almost-identical versions.
Fixes https://github.com/flutter/flutter/issues/53223
The JSON codec is awkward to use in the wrapper (since the client has to build and link one of the JSON libraries to do so). Since it would be very cumbersome to wrap in a C API, and there's essentially no reason to use it instead of the standard codec, this removes it from the wrapper entirely.
Since some system channels (internal to the engine) still use it, it's moved into common/cpp instead of being eliminated entirely. Internally we always use RapidJSON though, so the jsoncpp implementation is removed. Also adds some unit test coverage, since there wasn't any.
Fixes#30669
