fix(share_plus): downsample iOS share previews off main thread

[ananya-kaul]

Summary

• Generate image previews with ImageIO thumbnail downsampling on a utility queue before presenting the iOS share sheet.
• Reuse the cached thumbnail in the placeholder, thumbnail, and link metadata callbacks so those synchronous callbacks do not decode the full-size image.

Fixes #4001

Verification

• flutter test in packages/share_plus/share_plus — passed (14 tests, 1 skipped).
• iOS integration test on a simulator — passed locally (3 tests). A temporary iOS 15 deployment-target override was needed because the installed Xcode no longer accepts iOS 13 as the app target; the override was removed afterward.
• flutter build ios --no-codesign in the example app — passed with that same temporary deployment-target override.
• clang-format --dry-run --Werror and git diff --check — passed.
• GitHub's iOS integration job was cancelled at its 30-minute limit without reporting a test assertion failure.

[ananya-kaul]

Hi, this PR is ready for review. The iOS integration tests passed locally, but GitHub cancelled the iOS job at its 30-minute limit. Could a maintainer review the change and rerun that check?