feat(win32): support prebuilt assets in the build hook

[halildurmus]

Description

The build hook currently compiles native code from C source on the user's machine using package:native_toolchain_c, which requires MSVC to be installed. This is a significant friction point for users that don't have the necessary tools installed — even when they're not targeting Windows desktop themselves (e.g., they just have package:win32 as a transitive dependency).

This PR changes the build hook to download prebuilt binaries from the GitHub release artifacts instead of compiling from source, by default.

Key details

• Prebuilt .dll assets are downloaded from the corresponding GitHub release tag at build time
• Downloads are verified by MD5 hash to detect corruption or partial transfers
• Retry logic with exponential backoff handles transient network failures
• Users who prefer or require local compilation can opt out via the local_build user-define in their pubspec.yaml:

hooks:
  user_defines:
    win32:
      local_build: true

Related Issue

fluttercommunity/plus_plugins#3794

Type of Change

• 🚀 feat – New feature (non-breaking change that adds functionality)
• 🛠️ fix – Bug fix (non-breaking change that fixes an issue)
• ❌ ! – Breaking change (fix or feature that causes existing functionality to change)
• ⚡ perf – Performance improvement
• 🧹 refactor – Code refactor (no functionality change)
• 📝 docs – Documentation update
• 🎨 style – Code style changes (formatting, renaming, etc.)
• 🧪 test – Test update or addition
• 🔧 build – Build related changes
• ✅ ci – CI related changes
• 🗑️ chore – Chore (maintenance, non-production code change)
• ◀️ revert – Revert a previous commit

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.92%. Comparing base (7569f7a) to head (e582919).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
+ Coverage   85.69%   87.92%   +2.23%     
==========================================
  Files          91      216     +125     
  Lines        5202    10530    +5328     
==========================================
+ Hits         4458     9259    +4801     
- Misses        744     1271     +527     

Flag	Coverage Δ
ffi_leak_tracker	83.96% <ø> (ø)
generator	86.09% <ø> (+<0.01%)	⬆️
win32	80.07% <ø> (ø)
win32_clipboard	93.00% <ø> (ø)
win32_registry	88.53% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 126 files with indirect coverage changes

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