feat: Add URL caching and orphaned file cleanup

[abdelbaki1]

Good evening,
i have been using this library for quite few time and i 'd like to share this PR which introduces several improvements to the django-minio-backend:

URL Caching

• Added caching layer for pre-signed URLs to improve performance
• Configurable cache timeout and prefix
• Automatic cache invalidation based on ETag
• Skips caching for public buckets

Orphaned File Management

• New management command to clean orphaned files from MinIO
• Option to check for missing files referenced in database
• Dry-run mode to preview changes
• Improved error handling and logging

Documentation

• Updated README with new features and configuration options

[Alireza2n]

Nice work @abdelbaki1.
I can help with test coverage if @theriverman is onboard with merging the PR.

[abdelbaki1]

I am already using this on my projects and i forgot about adding the tests, appreciate your help