Add experimental/pedometer (#1587)

* Add `experimental/pedometer`

* Fixup for linter warnings

* Update CI config

experimental/pedometer/README.md

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+# FFIgen + JNIgen pedometer
+
+This is a demo for some of our tooling around calling platform APIs directly from dart code. This repository represents a demo of a plugin that leverages FFIgen & JNIgen. There is also an example pedometer app that uses the bindings generated from these tools. 
+
+- [FFIgen](https://pub.dev/packages/ffigen) is used to generate bindings for C, Objective-C and Swift APIs
+- [JNIgen](https://pub.dev/packages/jnigen) is used to generate bindings for Jave and Kotlin APIs
+
+**These tools are both experimental and are currently a work in progress.** If you find any issues or have feedback, please file it on the corresponding Github repositories. 
+
+
+## Re-generating bindings
+The bindings that allow the dart code to call the platform code have already been generated [here](/lib). You can regenerate them by following the steps below. 
+
+### FFIgen 
+Configuration for FFIgen is [here](/ffigen.yaml) for the [CoreMotion](https://developer.apple.com/documentation/coremotion) framework. FFIgen currently does not support autogenerating the code to handle callbacks. So, there were a few steps needed to appropriately handle callbacks in Objective-C. You can [read more here](https://dart.dev/guides/libraries/objective-c-interop#callbacks-and-multithreading-limitations).
+
+`dart run ffigen --config ffigen.yaml`
+
+### JNIgen
+Configuration for JNIgen is [here](/jnigen.yaml) for the [HealthConnect API](https://developer.android.com/guide/health-and-fitness/health-connect). Right now, JNIgen requires an Android folder to generate the bindings so there are a few steps to regenerate:
+
+1. `flutter create .`  
+2. `flutter build apk`
+3. `dart run jnigen --config jnigen.yaml`
+4. delete the android folder
+
+
+## Running the example app
+Note that step counting is only available on physical devices. 
+
+### iOS
+- `flutter run`
+- Allow *pedometer* app access to step counting
+
+### Android
+- Make sure that Google Fit is installed (to ensure that steps are being counted)
+- `flutter run`
+- Install Health Connect and grant access to Google Fit and the *jni_demo* app
+
+
+## Project stucture
+
+* `src`: Contains the native source code, and a CmakeFile.txt file for building
+  that source code into a dynamic library.
+
+* `lib`: Contains the Dart code that defines the API of the plugin, and which
+  calls into the native code using `dart:ffi`.
+
+* platform folders (`ios` etc.): Contains the build files
+  for building and bundling the native code library with the platform application.
+
+* `example`: Contains the native source code, and a CmakeFile.txt file for building
+  that source code into a dynamic library.
+
+
+