Feature/added method to get current disk cache size

[kothawade29]

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Summary:

This PR adds a new method getDiskCacheSize() to both Android and iOS platforms for the FastImage library. The motivation is to give developers the ability to inspect how much disk space is currently used by image caching.

This feature is especially useful for apps with constrained storage budgets, for debugging cache behavior, and for implementing custom cache-clearing logic or storage warnings.

On iOS, it uses SDImageCache.diskCachePath and efficiently calculates file sizes using NSDirectoryEnumerator with preloaded metadata. On Android, it walks through the Glide cache directory recursively, summing file sizes in a background thread.

Changelog:

[ANDROID] [ADDED] - Added getDiskCacheSize() method to calculate Glide cache usage

[IOS] [ADDED] - Added getDiskCacheSize() method to calculate SDWebImage cache usage with NSURL-based traversal
Example:

Test Plan:

Test Video : https://drive.google.com/file/d/1aea-Zj8J2sHUfqQkLWe_VdzSrd3JDIHv/view?usp=drive_link

Functional Tests
TC-001: Basic Cache Size Retrieval
Objective: Verify method returns current disk cache size
Steps:
Clear disk cache
Load 3-5 images
Call getDiskCacheSize()
Expected: Returns size > 0 with valid bytes and MB values

TC-002: Empty Cache Size
Objective: Verify method handles empty cache correctly
Steps:
Clear all caches
Call getDiskCacheSize() immediately
Expected: Returns { diskCacheSizeBytes: 0, diskCacheSizeMB: 0 }

TC-003: Cross-Platform Consistency
Objective: Verify consistent behavior across platforms
Steps:
Load identical set of images on iOS and Android
Call getDiskCacheSize() on both platforms
Expected: Similar cache sizes (within reasonable variance)

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[JatinDream11]

Please resolve the conflicts and add testing example in ReactNativeFastImageExample.