feat: support custom HTTP client injection

[Seraphim200001]

Ticket: #6
Allow passing a pre-configured HTTP client (e.g. Guzzle) to the Client. This enables reuse of middleware stacks (e.g. Laravel retry/cache) instead of forcing the internal cURL transport.

📋 Description

This PR adds support for injecting a custom HTTP client into the SDK Client.

Previously, all requests were executed via an internal cURL implementation. This made it impossible to reuse existing HTTP client configurations and middleware stacks (e.g. Laravel retry() or cache()), as they were bypassed entirely.

With this change, users can now provide a pre-configured HTTP client (such as Guzzle or any PSR-18 compatible client). This allows requests to be executed through the application's existing HTTP layer, enabling middleware reuse, connection pooling, and consistent configuration.

The default behavior remains unchanged: if no custom client is provided, the SDK continues to use its internal cURL transport.

This improves flexibility while keeping the SDK lightweight and framework-agnostic.

✨ Type of Change

• 🐛 Bug fix (fixes an issue)
• ✨ New feature (adds functionality)
• 🔧 Refactoring (code restructuring without functional changes)
• 📚 Documentation (updates to documentation)
• 🧪 Tests (adding or modifying tests)
• 🔒 Security (security-related fixes)
• ⚡ Performance (performance improvements)
• 🎨 Style (formatting changes, no logic changes)

🔍 Main Changes

• Introduced HttpTransportInterface to abstract the HTTP layer
• Extracted existing cURL logic into a dedicated CurlTransport (default behavior unchanged)
• Updated Client to accept:
  • a custom transport implementing HttpTransportInterface
    or a PSR-18 compatible HTTP client (e.g. Guzzle)
  • Enabled reuse of pre-configured HTTP clients and middleware stacks (e.g. Laravel retry/cache)
  • Added documentation for custom HTTP client usage (README)
  • Added unit tests to verify transport injection and delegation behavior

🧪 Testing

• I have tested the changes locally
• I have added/updated unit tests
• All tests pass (./vendor/bin/phpunit)
• I have verified PHP 8.0+ compatibility

📝 Additional Notes

Added support for psr/http-client to allow integration with PSR-18 compatible HTTP clients (e.g. Guzzle)
No framework-specific dependencies were introduced
Guzzle support is implicit via PSR-18 and remains optional
Default cURL transport remains unchanged and is used when no custom client is provided

🔗 Related Issue

Closes #6

📸 Screenshots (if applicable)

✅ Checklist

• My code follows the project conventions
• I have performed a self-review of my code
• I have commented complex code where necessary
• Documentation has been updated (if needed)
• I have not introduced breaking changes (or documented them)
• I have verified no sensitive information is in the code