Make FlyteFile and FlyteDirectory pickleable (flyteorg#3030)

* make _downloader function in FlyteFile/Directory pickleable

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* make FlyteFile and Directory pickleable

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* remove unnecessary helper functions

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* fix lint

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* use partials instead of lambda

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* fix lint

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* remove unneeded helper function

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* update FlyteFilePathTransformer.downloader method

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* remove downloader staticmethod

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

* fix lint

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>

---------

Signed-off-by: Niels Bantilan <niels.bantilan@gmail.com>
Signed-off-by: Shuying Liang <shuying.liang@gmail.com>

flytekit/types/file/file.py

@@ -308,7 +308,8 @@ def __init__(
         if ctx.file_access.is_remote(self.path):
             self._remote_source = self.path
             self._local_path = ctx.file_access.get_random_local_path(self._remote_source)
-            self._downloader = lambda: FlyteFilePathTransformer.downloader(
+            self._downloader = partial(
+                ctx.file_access.get_data,
                 ctx=ctx,
                 remote_path=self._remote_source,  # type: ignore
                 local_path=self._local_path,
@@ -741,18 +742,6 @@ async def async_to_python_value(
         ff._remote_source = uri
         return ff
 
-    @staticmethod
-    def downloader(
-        ctx: FlyteContext, remote_path: typing.Union[str, os.PathLike], local_path: typing.Union[str, os.PathLike]
-    ) -> None:
-        """
-        Download data from remote_path to local_path.
-
-        We design the downloader as a static method because its behavior is logically
-        related to this class but don't need to interact with class or instance data.
-        """
-        ctx.file_access.get_data(remote_path, local_path, is_multipart=False)
-
     def guess_python_type(self, literal_type: LiteralType) -> typing.Type[FlyteFile[typing.Any]]:
         if (
             literal_type.blob is not None