Support _DynamicListReader in _setDynamicField

See the test for an explanation.

Note that I'm not sure what the purpose of `_setDynamicFieldWithField` and
`_setDynamicFieldStatic` is. It does not appear to be used. I've kept them for
now (they are a public API), but perhaps this can be removed.

test/test_capability.capnp

@@ -28,6 +28,8 @@ interface TestInterface {
   bar @1 () -> ();
   buz @2 (i: TestSturdyRefHostId) -> (x: Text);
   bam @3 (i :UInt32, j :Bool) -> (x: Text, i:UInt32);
+  bak1 @4 () -> (i:List(UInt32));
+  bak2 @5 (i:List(UInt32)) -> ();
   # baz @2 (s: TestAllTypes);
 }