[fs_connector][feat]: Add Python backend implementation for fs backend of offloading connector (fs_connector)

[kfirtoledo]

This PR adds the complete Python backend implementation for the filesystem offloading connector.
Included in this PR:
Adds the Python package modules:

• factory.py - registers the fs connector spec (SharedStorageOffloadingSpec)
• manager.py - implements the OffloadingManager logic for the filesystem backend
• mediums.py - defines the storage medium used by the connector
• spec.py - defines the filesystem storage backend class used by vLLM
• worker.py - executes asynchronous file I/O tasks for reading and writing blocks

Adds project configuration:

• pyproject.toml
• README.md
• docs/

Adds initial test coverage:

• tests/test_fs_backend.py

Key capabilities:
Provides the Python interface for the filesystem connector used by the offloading subsystem.
Implements connector registration, scheduling logic, and storage medium abstraction.
Connects cleanly to the underlying C++ and CUDA extension modules.
Defines SharedStorageOffloadingSpec for integration with vLLM's KV connector framework.
This module completes the high-level Python layer required for end-to-end operation of the filesystem offloading connector.

More details can be found in the README file.