Add walk operator

[tchaton]

What does this PR do?

This PR introduces a walk operator to accelerate files listing on Lightning AI.
The walk operator queues listing calls into a ThreadPoolExecutor.

Note: The order of files and folders isn't depth-first anymore due to the asynchronous call.

Fixes #<issue_number>

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• Did you make sure to update the documentation with your changes? (if necessary)
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• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
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cc @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 lightning_data: CPU workflow

Check ID	Status
data-cpu (macOS-11, lightning, 3.10, 2.1)	success	✅
data-cpu (ubuntu-20.04, lightning, 3.10, 2.1)	success	✅
data-cpu (windows-2022, lightning, 3.10, 2.1)	success	✅

These checks are required after the changes to src/lightning/data/__init__.py, src/lightning/data/streaming/functions.py, tests/tests_data/streaming/test_functions.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/data/__init__.py, src/lightning/data/streaming/functions.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	success	✅
install-pkg (ubuntu-22.04, app, 3.11)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.8)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.11)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.8)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.11)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.8)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.11)	success	✅
install-pkg (ubuntu-22.04, notset, 3.8)	success	✅
install-pkg (ubuntu-22.04, notset, 3.11)	success	✅
install-pkg (macOS-12, app, 3.8)	success	✅
install-pkg (macOS-12, app, 3.11)	success	✅
install-pkg (macOS-12, fabric, 3.8)	success	✅
install-pkg (macOS-12, fabric, 3.11)	success	✅
install-pkg (macOS-12, pytorch, 3.8)	success	✅
install-pkg (macOS-12, pytorch, 3.11)	success	✅
install-pkg (macOS-12, lightning, 3.8)	success	✅
install-pkg (macOS-12, lightning, 3.11)	success	✅
install-pkg (macOS-12, notset, 3.8)	success	✅
install-pkg (macOS-12, notset, 3.11)	success	✅
install-pkg (windows-2022, app, 3.8)	success	✅
install-pkg (windows-2022, app, 3.11)	success	✅
install-pkg (windows-2022, fabric, 3.8)	success	✅
install-pkg (windows-2022, fabric, 3.11)	success	✅
install-pkg (windows-2022, pytorch, 3.8)	success	✅
install-pkg (windows-2022, pytorch, 3.11)	success	✅
install-pkg (windows-2022, lightning, 3.8)	success	✅
install-pkg (windows-2022, lightning, 3.11)	success	✅
install-pkg (windows-2022, notset, 3.8)	success	✅
install-pkg (windows-2022, notset, 3.11)	success	✅

These checks are required after the changes to src/lightning/data/__init__.py, src/lightning/data/streaming/functions.py.

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Thank you for your contribution! 💜

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[justusschock]

some comments, but looks ok to me