Add support for getting argument count from Callables

test/project/main.gd

@@ -102,6 +102,7 @@ func _ready():
 	# mp_callable() with void method.
 	var mp_callable: Callable = example.test_callable_mp()
 	assert_equal(mp_callable.is_valid(), true)
+	assert_equal(mp_callable.get_argument_count(), 3)
 	mp_callable.call(example, "void", 36)
 	assert_equal(custom_signal_emitted, ["unbound_method1: Example - void", 36])
 
@@ -117,14 +118,17 @@ func _ready():
 
 	# mp_callable() with return value.
 	var mp_callable_ret: Callable = example.test_callable_mp_ret()
+	assert_equal(mp_callable_ret.get_argument_count(), 3)
 	assert_equal(mp_callable_ret.call(example, "test", 77), "unbound_method2: Example - test - 77")
 
 	# mp_callable() with const method and return value.
 	var mp_callable_retc: Callable = example.test_callable_mp_retc()
+	assert_equal(mp_callable_retc.get_argument_count(), 3)
 	assert_equal(mp_callable_retc.call(example, "const", 101), "unbound_method3: Example - const - 101")
 
 	# mp_callable_static() with void method.
 	var mp_callable_static: Callable = example.test_callable_mp_static()
+	assert_equal(mp_callable_static.get_argument_count(), 3)
 	mp_callable_static.call(example, "static", 83)
 	assert_equal(custom_signal_emitted, ["unbound_static_method1: Example - static", 83])
 
@@ -140,6 +144,7 @@ func _ready():
 
 	# mp_callable_static() with return value.
 	var mp_callable_static_ret: Callable = example.test_callable_mp_static_ret()
+	assert_equal(mp_callable_static_ret.get_argument_count(), 3)
 	assert_equal(mp_callable_static_ret.call(example, "static-ret", 84), "unbound_static_method2: Example - static-ret - 84")
 
 	# CallableCustom.
@@ -150,6 +155,7 @@ func _ready():
 	assert_equal(custom_callable.hash(), 27);
 	assert_equal(custom_callable.get_object(), null);
 	assert_equal(custom_callable.get_method(), "");
+	assert_equal(custom_callable.get_argument_count(), 2)
 	assert_equal(str(custom_callable), "<MyCallableCustom>");
 
 	# PackedArray iterators

test/src/example.cpp

@@ -49,6 +49,11 @@ public:
 		return ObjectID();
 	}
 
+	virtual int get_argument_count(bool &r_is_valid) const {
+		r_is_valid = true;
+		return 2;
+	}
+
 	virtual void call(const Variant **p_arguments, int p_argcount, Variant &r_return_value, GDExtensionCallError &r_call_error) const {
 		r_return_value = "Hi";
 		r_call_error.error = GDEXTENSION_CALL_OK;