Add REST API support with backward-compatible auto detection

[espipj]

Summary

This PR adds support for newer Sagemcom firmware that exposes /api/v1/* endpoints, while preserving compatibility with legacy /cgi/json-req firmware.

What Changed

• Added ApiMode enum:
  • ApiMode.AUTO (default)
  • ApiMode.LEGACY
  • ApiMode.REST
• Extended SagemcomClient with api_mode parameter.
• Implemented API mode selection logic in login():
  • LEGACY: force legacy flow
  • REST: force REST flow
  • AUTO: try legacy first, fallback to REST when legacy endpoint appears unavailable.
• Added REST request path for:
  • login() via POST /api/v1/login (form-encoded)
  • logout() via POST /api/v1/logout
  • get_device_info() via GET /api/v1/device
  • get_hosts() via GET /api/v1/home
• Kept legacy behavior for XPath-based methods; these now raise clear NotImplementedError in REST mode.

Backward Compatibility

• Existing usage remains compatible.
• Default mode is AUTO, so legacy users do not need to change code.
• Explicit mode can be set when deterministic behavior is required.

Tests

• Added unit test for AUTO fallback to REST on legacy 503 HTML response.
• Added unit test for REST get_hosts() parsing (wireless + ethernet devices).

Documentation

• README updated with:
  • ApiMode usage in quickstart
  • API mode behavior and REST/legacy limitations