Add preheat function for OCI image blob preheating via Dragonfly proxy

[Copilot]

• Create new dragonfly-client-util/examples/request/ package with two binaries (get and preheat)
• Create dragonfly-client-util/examples/request/Cargo.toml with [[bin]] entries for both get and preheat
• Move get/src/main.rs → request/src/bin/get.rs and preheat/src/main.rs → request/src/bin/preheat.rs
• Create a combined README.md for the request package
• Remove old dragonfly-client-util/examples/get/ and dragonfly-client-util/examples/preheat/ directories
• Update workspace Cargo.toml to replace the two old members with the new one
• Verify lint and check passes

[codecov]

Codecov Report

❌ Patch coverage is 0% with 157 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.41%. Comparing base (5400d49) to head (8b9c34f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client-util/src/request/mod.rs	0.00%	157 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1774      +/-   ##
==========================================
- Coverage   44.54%   44.41%   -0.14%     
==========================================
  Files          91       91              
  Lines       25834    26054     +220     
==========================================
+ Hits        11508    11571      +63     
- Misses      14326    14483     +157     

Files with missing lines	Coverage Δ
dragonfly-client-util/src/request/mod.rs	32.82% <0.00%> (-11.07%)	⬇️

... and 2 files with indirect coverage changes

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[BraveY]

LGTM