feat: add model-first payload entities and transport guidance

[sudiptpa]

Highlights

• Added model-first payment payload support for cleaner, safer request building:
  • CustomerInfo
  • AmountBreakdownItem
  • ProductDetail
  • upgraded EpaymentInitiateRequest with make(...), getters/setters, and fluent adders
• Preserved backward compatibility for initiate payloads by still accepting array-based fields in EpaymentInitiateRequest.
• Added readable payment API aliases:
  • payments()->initiate(...) (alias of create(...))
  • payments()->lookup(...) (alias of status(...))
• Kept CurlTransport as default core transport and improved the runtime error when ext-curl is missing:
  • now clearly instructs users to install ext-curl or provide a custom TransportInterface.
• Expanded tests:
  • new unit tests for model serialization and backward-compatible array payloads
  • new unit tests for initiate()/lookup() aliases
• Upgraded docs (README.md) with:
  • concrete PSR-18/Guzzle transport adapter example using Khalti SDK naming
  • clearer custom transport guidance
  • explicit optional-dependency note for PSR-18/Guzzle integrations
  • restored typed model/value object highlights
• Updated architecture docs (ARCHITECTURE.md) to reflect:
  • model-first payload flow
  • alias method flow
  • default transport behavior and override path
• Dependency/CI alignment:
  • removed ext-json hard dependency from composer.json
  • synced composer.lock so composer validate --strict passes in CI