ci: add binary-compatibility-validator plugin

[nicklasl]

This PR adds a validation step to ensure that we do not change the public API without noticing.

The gradle check step (runs on CI builds) will validate the current API of the classes in the gradle project against the checked in file (confidence/api/confidence.api or Provider/api/Provider.api). If it detects changes in the API, it will fail like this:
I added a String parameter to the flush function

> Task :Confidence:apiCheck FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':Confidence:apiCheck'.
> API check failed for project Confidence.
  --- /Users/nicklasl/src/confidence/confidence-openfeature-provider-kotlin/Confidence/api/Confidence.api
  +++ /Users/nicklasl/src/confidence/confidence-openfeature-provider-kotlin/Confidence/build/api/Confidence.api
  @@ -35,7 +35,7 @@
        public final fun asyncFetch ()V
        public final fun awaitReconciliation (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
        public final fun fetchAndActivate (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
  -     public fun flush ()V
  +     public fun flush (Ljava/lang/String;)V
        public fun getContext ()Ljava/util/Map;
        public final fun getFlag (Ljava/lang/String;Ljava/lang/Object;)Lcom/spotify/confidence/Evaluation;
        public final fun getValue (Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/Object;
  @@ -428,7 +428,7 @@
   }

   public abstract interface class com/spotify/confidence/EventSender : com/spotify/confidence/Contextual {
  -     public abstract fun flush ()V
  +     public abstract fun flush (Ljava/lang/String;)V
        public abstract fun stop ()V
        public abstract fun track (Lcom/spotify/confidence/EventProducer;)V
        public abstract fun track (Ljava/lang/String;Ljava/util/Map;)V

  You can run :Confidence:apiDump task to overwrite API declarations

If the change was intended, you would run ./gradlew confidence:apiDump to re-generate the "golden file" in the api/ folder and amend that file with your commit.

[andreas-karlsson]

Nice!

[fabriziodemaria]

Nice with the added docs ⭐️