Adds the `namespace` property necessary for AGP 8 compatibility to the plugin templates, with the conditional logic to ensure that it doesn't break AGP <4.2, so that new plugins will be maximally compatible.
Part of https://github.com/flutter/flutter/issues/125181
* Building shared C source code as part of the native build for platforms Android, iOS, Linux desktop, MacOS desktop, and Windows desktop.
* Sample code doing a synchronous FFI call.
* Sample code doing a long running synchronous FFI call on a helper isolate.
* Use of `package:ffigen` to generate the bindings.
