add double hook interface

Signed-off-by: Michael Beemer <beeme1mr@users.noreply.github.com>

src/main/java/dev/openfeature/javasdk/DoubleHook.java

@@ -0,0 +1,9 @@
+package dev.openfeature.javasdk;
+
+public interface DoubleHook extends Hook<Double> {
+
+    @Override
+    default boolean supportsFlagValueType(FlagValueType flagValueType) {
+        return FlagValueType.DOUBLE == flagValueType;
+    }
+}

src/test/java/dev/openfeature/javasdk/fixtures/HookFixtures.java

@@ -18,6 +18,10 @@ public interface HookFixtures {
         return spy(IntegerHook.class);
     }
 
+    default Hook<Double> mockDoubleHook() {
+        return spy(DoubleHook.class);
+    }
+
     default Hook<?> mockGenericHook() {
         return spy(Hook.class);
     }