feat: expose PackageCache in py-rattler

[DCchoudhury15]

Description

PR #1003 added a layered package cache to the Rust core but it was never exposed to Python. This PR adds the Python bindings for PackageCache and ValidationMode.

Users can now do:

from rattler import PackageCache, ValidationMode
from pathlib import Path

cache = PackageCache(Path("/tmp/cache"))

# or with multiple layers
cache = PackageCache.new_layered(
    [Path("/slow/cache"), Path("/fast/cache")],
    validation_mode=ValidationMode.Fast,
)

cache.paths()
cache.writable_paths()
cache.readonly_paths()

Fixes #632

How Has This Been Tested?

Added 12 unit tests in py-rattler/tests/unit/test_package_cache.py.

cd py-rattler && pytest tests/unit/test_package_cache.py -v

AI Disclosure

• This PR contains AI-generated content.(testfile has been written with the help of ai)
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added sufficient tests to cover my changes.