Add registry abstraction layer for improved maintainability

[kaovilai]

Implements a clean abstraction layer between udistribution and
distribution/distribution to reduce coupling and improve maintainability
when external dependencies are updated.

Key components:

• pkg/registry/interface.go: Core Registry interface and config abstractions
• pkg/registry/distribution/adapter.go: Distribution adapter implementation
• pkg/registry/errors.go: Error abstraction layer
• pkg/client/client_v2.go: New client using registry abstractions
• pkg/client/client_v2_test.go: Comprehensive tests for new client
• pkg/examples/abstraction_example.go: Usage examples

Benefits:

• Version independence: Only adapter needs updates when distribution changes
• Clean separation: Business logic separated from implementation details
• Testability: Interface allows for mock implementations
• Backward compatibility: Existing client.NewClient() continues to work
• Error consistency: Unified error handling across the library

Migration path: New projects should use client.NewClientV2() while
existing code continues to work unchanged.

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[openshift-ci]

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