fix(ios): honor cacheType and headers in ImagePipeline prefetch - #69

Problem

Two iOS prefetch regressions vs. the 4.x line:

1. prefetchToCacheType ignores its cacheType argument. SDWebImagePrefetcher's default context stores to disk+memory either way, so a disk prefetch also decodes and warms memory — diverging from Android's bytes-only Glide.downloadOnly behavior.
2. getContextFromOptions drops PrefetchOptions.headers because it never sets SDWebImageContextDownloadRequestModifier, so prefetch({ headers }) silently sends no headers on iOS while Android's CustomGlideUrl forwards them.

Fix

Ports the 4.x implementation forward:

• prefetchToCacheType sets SDWebImageContextStoreCacheType on the prefetch context, plus SDWebImageContextOriginalStoreCacheType so a memory prefetch does not spill original bytes to disk.
• getContextFromOptions applies options.headers through an SDWebImageDownloaderRequestModifier — same pattern as the existing Img.headers handling in updateView, which also fixes headers for getCacheKey and the other prefetch callers.

Upstream-relevant note: 4.x additionally allowed headers as a () => Record<string, string> thunk; 5.x types it Record<string, string> only, so this uses the plain form.