engine/shell/platform/common/cpp/client_wrapper

This code is intended to be built into plugins and applications to provide
higher-level, C++ abstractions for interacting with the Flutter library.

Over time, the goal is to move more of this code into the library in a way that
provides a usable ABI (e.g., does not use standard library in the interfaces).

Note that this wrapper is still in early stages. Expect significant churn in
both the APIs and the structure of the wrapper (e.g., the exact set of files
that need to be built).