a new Feature Flag API

Feature flags are intended to be used when a new feature is added to 
filament and have generally two purposes:

1) during feature development, the feature can be implemented but
   disabled which can help developing large features. This way the
   feature can be tested by stakeholders while it's being developed
   without impacting other clients.

2) once a feature is ready, its flag can be enabled by default, but 
   in case the feature breaks something or has unintended consequence,
   clients have the option to turn it off.

Feature flags are intended to have a relatively short life span, i.e.
once a feature is stable, the flag fill be removed.

There two types of feature flags. Constant feature flags can only be
set during Engine initialization via Engine::Builder. Non-constant
feature flags can be set at any time. 

Feature flags SHOULD NOT be used as configuration or settings.


Feature flags are designed with a few ideas in mind:
- they are very cheap to check inside the engine
- non-constant flags can easily be toggled using ImGUI

android/filament-android/src/main/cpp/Engine.cpp

@@ -499,6 +499,37 @@ Java_com_google_android_filament_Engine_nGetActiveFeatureLevel(JNIEnv *, jclass,
     return (jint)engine->getActiveFeatureLevel();
 }
 
+extern "C"
+JNIEXPORT jboolean JNICALL
+Java_com_google_android_filament_Engine_nHasFeatureFlag(JNIEnv *env, jclass clazz,
+        jlong nativeEngine, jstring name_) {
+    Engine* engine = (Engine*) nativeEngine;
+    const char *name = env->GetStringUTFChars(name_, 0);
+    std::optional<bool> result = engine->getFeatureFlag(name);
+    env->ReleaseStringUTFChars(name_, name);
+    return result.has_value();
+}
+extern "C"
+JNIEXPORT jboolean JNICALL
+Java_com_google_android_filament_Engine_nSetFeatureFlag(JNIEnv *env, jclass clazz,
+        jlong nativeEngine, jstring name_, jboolean value) {
+    Engine* engine = (Engine*) nativeEngine;
+    const char *name = env->GetStringUTFChars(name_, 0);
+    jboolean result = engine->setFeatureFlag(name, (bool)value);
+    env->ReleaseStringUTFChars(name_, name);
+    return result;
+}
+extern "C"
+JNIEXPORT jboolean JNICALL
+Java_com_google_android_filament_Engine_nGetFeatureFlag(JNIEnv *env, jclass clazz,
+        jlong nativeEngine, jstring name_) {
+    Engine* engine = (Engine*) nativeEngine;
+    const char *name = env->GetStringUTFChars(name_, 0);
+    std::optional<bool> result = engine->getFeatureFlag(name);
+    env->ReleaseStringUTFChars(name_, name);
+    return result.value_or(false); // we should never fail here
+}
+
 extern "C" JNIEXPORT jlong JNICALL Java_com_google_android_filament_Engine_nCreateBuilder(JNIEnv*,
         jclass) {
     Engine::Builder* builder = new Engine::Builder{};
@@ -565,6 +596,16 @@ extern "C" JNIEXPORT void JNICALL Java_com_google_android_filament_Engine_nSetBu
     builder->paused((bool) paused);
 }
 
+extern "C"
+JNIEXPORT void JNICALL
+Java_com_google_android_filament_Engine_nSetBuilderFeature(JNIEnv *env, jclass clazz,
+        jlong nativeBuilder, jstring name_, jboolean value) {
+    Engine::Builder* builder = (Engine::Builder*) nativeBuilder;
+    const char *name = env->GetStringUTFChars(name_, 0);
+    builder->feature(name, (bool)value);
+    env->ReleaseStringUTFChars(name_, name);
+}
+
 extern "C" JNIEXPORT jlong JNICALL
 Java_com_google_android_filament_Engine_nBuilderBuild(JNIEnv*, jclass, jlong nativeBuilder) {
     Engine::Builder* builder = (Engine::Builder*) nativeBuilder;