[FEATURE] Add Data API support with routing metadata headers and pagination

[danny-beton-lrn]

Summary

Adds comprehensive Data API support to the Ruby SDK, following the pattern established in the Node.js SDK.

Changes

Core Implementation

• New DataApi class (lib/learnosity/sdk/request/data_api.rb)
  • request() - Make single authenticated Data API requests
  • request_iter() - Iterate through paginated responses (returns Enumerator)
  • results_iter() - Iterate through individual results across pages (returns Enumerator)
  • Automatic routing metadata headers: X-Learnosity-Consumer, X-Learnosity-Action, X-Learnosity-SDK

Rails Quickstart Integration

• Added Data API demo to quickstart application
  • Controller: app/controllers/data_api_controller.rb
  • View: app/views/data_api/index.html.erb
  • Route: /data_api/index
  • Demonstrates all three iteration methods with live API calls

Ruby 2.6 Compatibility Fixes

• Commented out spring and spring-watcher-listen gems (require Ruby 2.7+)
• Added require 'logger' to config/boot.rb for Rails 6.1 compatibility
• Updated README to document Ruby 2.6+ support

Testing

• Unit tests: spec/learnosity/sdk/request/data_api_spec.rb
• Integration tests: spec/integration/data_api_spec.rb
• Example usage: examples/simple/data_api_example.rb

Documentation

• Updated REFERENCE.md with Data API usage examples
• Updated README.md with Ruby version compatibility notes

Testing

All existing tests pass. New tests added for Data API functionality.