Add asset transformation sample (#2267)

This adds a sample Flutter project that demonstrates a
soon-to-be-released feature, asset transformation[^1]. [PR for
flutter.dev
documentation](https://github.com/flutter/website/pull/10471).

This feature isn't the easiest to explain using documentation, so I
think augmenting that documentation with a sample is appropriate.

This sample demonstrates 1) how to use an existing Dart package (that is
compatible with the feature) as an asset transformer and 2) how to write
a Dart package that is compatible with this feature. This should be
clear from the README.md.

**Advice for reviewing this PR.** The goal here is that most users that
read the documentation and follow the link from there to this sample
should be able to figure out what the feature does and how to use it.
Try to imagine yourself in this position and follow this story. If the
feature is still unclear, then there is probably something we can do to
improve this sample or the docs. Said more simply, follow these steps:
1) Start at the new section to be added to Flutter.dev
(https://flutter-docs-prod--pr10471-document-asset-transformers-cc21qf01.web.app/ui/assets/assets-and-images#automatic-transformation-of-asset-files-at-build-time).
It should naturally link you to the sample project. Start with the
README and see if things make sense.

## Pre-launch Checklist

- [X] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [X] I signed the [CLA].
- [X] I read the [Contributors Guide].
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

[^1]: If you are super curious about this feature, see [the tracking
issue for its
implementation](https://github.com/flutter/flutter/issues/143348).

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

asset_transformation/macos/Runner/Configs/AppInfo.xcconfig

@@ -0,0 +1,14 @@
+// Application-level settings for the Runner target.
+//
+// This may be replaced with something auto-generated from metadata (e.g., pubspec.yaml) in the
+// future. If not, the values below would default to using the project name when this becomes a
+// 'flutter create' template.
+
+// The application's name. By default this is also the title of the Flutter window.
+PRODUCT_NAME = asset_transformation
+
+// The application's bundle identifier
+PRODUCT_BUNDLE_IDENTIFIER = com.example.assetTransformation
+
+// The copyright displayed in application information
+PRODUCT_COPYRIGHT = Copyright © 2024 com.example. All rights reserved.

asset_transformation/macos/Runner/Configs/Debug.xcconfig

@@ -0,0 +1,2 @@
+#include "../../Flutter/Flutter-Debug.xcconfig"
+#include "Warnings.xcconfig"

asset_transformation/macos/Runner/Configs/Release.xcconfig

@@ -0,0 +1,2 @@
+#include "../../Flutter/Flutter-Release.xcconfig"
+#include "Warnings.xcconfig"

asset_transformation/macos/Runner/Configs/Warnings.xcconfig

@@ -0,0 +1,13 @@
+WARNING_CFLAGS = -Wall -Wconditional-uninitialized -Wnullable-to-nonnull-conversion -Wmissing-method-return-type -Woverlength-strings
+GCC_WARN_UNDECLARED_SELECTOR = YES
+CLANG_UNDEFINED_BEHAVIOR_SANITIZER_NULLABILITY = YES
+CLANG_WARN_UNGUARDED_AVAILABILITY = YES_AGGRESSIVE
+CLANG_WARN__DUPLICATE_METHOD_MATCH = YES
+CLANG_WARN_PRAGMA_PACK = YES
+CLANG_WARN_STRICT_PROTOTYPES = YES
+CLANG_WARN_COMMA = YES
+GCC_WARN_STRICT_SELECTOR_MATCH = YES
+CLANG_WARN_OBJC_REPEATED_USE_OF_WEAK = YES
+CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF = YES
+GCC_WARN_SHADOW = YES
+CLANG_WARN_UNREACHABLE_CODE = YES