[BUG] Make FlyteFile, FlyteDirectory, and StructuredDataset pickleable

[cosmicBboy]

Describe the bug

Currently, FlyteFile and FlyteDirectory objects with a non-default _downloader function (i.e. a _downloader that isn't the default noop function) cannot be pickled.

      File "/opt/micromamba/envs/runtime/lib/python3.11/multiprocessing/reduction.py", line 60, in dump
        ForkingPickler(file, protocol).dump(obj)
    AttributeError: Can't pickle local object 'FlyteDirToMultipartBlobTransformer.async_to_python_value.<locals>._downloader'

Message:

    AttributeError: Can't pickle local object 'FlyteDirToMultipartBlobTransformer.async_to_python_value.<locals>._downloader'

This error is raised when, e.g. users specify FlyteDirectory/File task inputs together with the Elastic plugin, which relies on pickling the inputs when running the task function code in parallel.

Expected behavior

This should just work.

Additional context to reproduce

No response

Screenshots

No response

Are you sure this issue hasn't been raised already?

• Yes

Have you read the Code of Conduct?

• Yes

[wild-endeavor]

@cosmicBboy does this issue impact structureddataset?

[cosmicBboy]

Oh, yeah I think it does

[wild-endeavor]

mind putting in the fix if it does?