Anim: add keytype argument to keyframe_insert() RNA function

Add an optional keyword argument `keytype` to the
`rna_struct.keyframe_insert()` function.

This makes it possible to set the new key's type. The code for this was
almost all in place, the only thing that was missing was the RNA
wrapper, which is what this commit adds.

Example: `bpy.context.object.keyframe_insert("location",
keytype='JITTER')`

There is no backward compatibility issue here, because the argument is
optional and defaults to the previously hardcoded value of `KEYFRAME`.

Pull Request: https://projects.blender.org/blender/blender/pulls/120578

tests/python/bl_animation_fcurves.py

@@ -188,6 +188,21 @@ class KeyframeInsertTest(AbstractAnimationTest, unittest.TestCase):
 
         bpy.ops.object.delete(use_global=False)
 
+    def test_keyframe_insert_keytype(self):
+        key_object = bpy.context.active_object
+
+        # Inserting a key with a specific type should work.
+        key_object.keyframe_insert("location", keytype='GENERATED')
+
+        # Unsupported/unknown types should be rejected.
+        with self.assertRaises(ValueError):
+            key_object.keyframe_insert("rotation_euler", keytype='UNSUPPORTED')
+
+        # Only a single key should have been inserted.
+        keys = key_object.animation_data.action.fcurves[0].keyframe_points
+        self.assertEqual(len(keys), 1)
+        self.assertEqual(keys[0].type, 'GENERATED')
+
     def test_keyframe_insertion_high_frame_number(self):
         bpy.ops.mesh.primitive_monkey_add()
         key_count = 100