* Add an integration test to plugin template example

Dart unit tests don't exercise host-side plugin code at all, so the
example tests in the plugin template currently have very little
meaningful coverage. This adds an integration test to the example app
when creating a plugin, so that there's an example of how to actually
test that a complete round-trip plugin call works.

This is done as a separate template that's currently only used by the
plugin template because I don't know what a good example for a
non-plugin case would be that isn't largely just a duplicate of the
widget tests. However, the integration test pre-includes conditionals
around the parts that are plugin-specific so that it can more easily be
expanded to other use cases later (e.g., in
https://github.com/flutter/flutter/issues/68818).

Part of https://github.com/flutter/flutter/issues/82458

* Add integration test to expected dependencies of a plugin app

* Test fixes

* Make an explicit test case

The recent survey suggests that some people are not finding documentation after they get started with Flutter, so let's just give them links right away. It might not help everyone but there's not really any downside so...

This reverts commit 3802eb62.

* Use single quotes for `dartSdkVersionBounds`

* Update update_packages_test.dart

* Update test_project.dart

* Update tests_project.dart

* Update compile_error_project.dart

* Update hot_reload_with_asset.dart

* Update break_on_framework_exceptions_test.dart

* Update basic_project.dart

This reverts commit 5257f029.

* 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.

Windows package identifiers are globally unique strings, typically a
GUID. These are required for templates that require a Windows package
name as described in
https://docs.microsoft.com/en-us/windows/win32/appxpkg/appx-portal

Fixes https://github.com/flutter/flutter/issues/82587