feat: add warning handler

[TerryHowe]

What this PR does / why we need it:

Add a warning handler and some tests for it. Gets a little bloat out of options/remote.go

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.36%. Comparing base (d19ca44) to head (3395408).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1856      +/-   ##
==========================================
+ Coverage   85.32%   85.36%   +0.03%     
==========================================
  Files         143      144       +1     
  Lines        6739     6730       -9     
==========================================
- Hits         5750     5745       -5     
+ Misses        703      700       -3     
+ Partials      286      285       -1     

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[Copilot]

Pull Request Overview

This PR introduces a centralized warning handler to manage registry warnings and reduces code duplication. The change extracts warning handling logic from the Remote struct into a dedicated WarningHandler component with proper concurrency handling.

• Adds a new WarningHandler type with thread-safe warning deduplication
• Removes warning handling code from the Remote struct to reduce bloat
• Provides comprehensive test coverage for the new warning handler functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
cmd/oras/internal/option/warning_handler.go	New warning handler implementation with sync.Map for thread-safe deduplication
cmd/oras/internal/option/warning_handler_test.go	Comprehensive test suite covering deduplication, concurrency, and multi-registry scenarios
cmd/oras/internal/option/remote.go	Removes old warning handling code and integrates new WarningHandler
cmd/oras/internal/option/binary_target.go	Removes manual warning map initialization that's no longer needed

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