Some (sadly breaking) improvements to the Fluid API (#6110)

* Some (sadly breaking) improvements to the API, after feedback and concerns provided by users:
  - Moved calculation of the translation key to the FluidAttributes constructor, so that builders can be shared between still & flowing sub-fluids.
  - Moved biome-based coloring to a dedicated FluidAttributes.Water variant, which is not used by default.
  - Added logic to automatically gather fluid textures into the list of textures to bake.
  - Patched BucketItem and FlowingFluidBlock to avoid eager access to the fluid objects.
  - Added a ForgeFlowingFluid class, as a more user-friendly way to construct a new FlowingFluid.

src/main/java/net/minecraftforge/fml/RegistryObject.java

@@ -35,7 +35,7 @@ import java.util.stream.Stream;
 
 import javax.annotation.Nullable;
 
-public final class RegistryObject<T extends IForgeRegistryEntry<? super T>>
+public final class RegistryObject<T extends IForgeRegistryEntry<? super T>> implements Supplier<T>
 {
     private final ResourceLocation name;
     @Nullable