Feature/clone function

[mosquito]

This pull request introduces a new asynchronous file cloning feature to the aiofile library, allowing multiple independent file-like objects to share a single file descriptor with separate offsets. This enables concurrent reading (and potentially writing) from the same file without reopening it, which can improve performance in certain scenarios. The change includes a new clone helper, updates to the core AIOFile class, and corresponding tests and documentation.

New file cloning feature:

• Added a clone helper function and FileIOCloner class to aiofile.utils, allowing users to create independent file-like clones from an existing file object. This enables concurrent operations with separate offsets on the same file descriptor.
• Updated the public API in aiofile/__init__.py to export clone and FileIOCloner. [1] [2] [3]

Core class enhancements:

• Modified the AIOFile class to support reference counting for clones, ensuring the file is only closed when all clones are released. Added a clone() async method and internal locking for thread safety. [1] [2]

Documentation:

• Added a new section to README.md explaining the clone helper, including a practical example and caveats for Windows users.

Testing:

• Added new tests in tests/test_aio.py to verify the behavior of file cloning, including correct reading, offset handling, and reference-counted closing. [1] [2]