[path_provider] Convert iOS/macOS to FFI package (#9762)

Converts `path_provider_foundation` from a Flutter plugin to a Dart-only package using `ffigen` to call into Foundation directly.

Most of the Dart unit tests had to be converted to integration tests, although they are still conceptually unit tests (i.e., they still inject mocks for the platform calls). This is because unlike Pigeon, where the API layer being mocked uses Dart types, the FFI calls that we now mock use Obj-C types, and those don't exist outside of an actual app runtime, so the integration test driver is the only place they can be hosted. This is very similar to how most of our web plugin implementations are unit tested.

Fixes flutter/flutter#111033

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Author: stuartmorgan-g

packages/path_provider/path_provider_foundation/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 2.5.0
+
+* Replaces Flutter-plugin-based implementation with direct FFI calls to
+  Foundation.
+
 ## 2.4.4
 
 * Updates to Pigeon 26.

packages/path_provider/path_provider_foundation/CONTRIBUTING.md

@@ -0,0 +1,18 @@
+# Contributing
+
+## `ffigen`
+
+This package uses [ffigen](https://pub.dev/packages/ffigen) to call Foundation
+methods, rather than using the standard Flutter plugin structure. To add new
+functionality to the FFI interface, update `tool/ffigen.dart`, then run:
+
+```bash
+dart run tool/ffigen.dart
+```
+
+### Configuration philosophy
+
+This package intentionally uses very strict filtering rules to include only the
+necessary methods and functions. This is partially to keep the package small,
+but mostly to avoid unnecessarily generating anything that requires native code
+helpers, which would require setting up a native compilation step.