Add support for evals

[alzeck]

This pull request adds support for the OpenAI Evals API introduced earlier this year.

Test for retrieving an output item are a bit flaky as it requires the evals run to be completed in order for the fetch to work, happy for any suggestion on how to implement this differently if needed.

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[Copilot]

Pull request overview

This pull request adds comprehensive support for the OpenAI Evals API, enabling users to systematically evaluate AI model performance through the ruby-openai gem. The implementation follows the existing codebase patterns with proper module structure, test coverage using VCR cassettes, and detailed documentation.

Key Changes:

• Added complete Evals API client implementation with support for evaluations, runs, and output items
• Comprehensive RSpec test suite with 13 test cases covering all API endpoints
• Extensive README documentation with usage examples for all supported operations

Reviewed changes

Copilot reviewed 35 out of 35 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/openai/evals.rb	New Evals module with nested Runs and OutputItems classes implementing all API endpoints
lib/openai/client.rb	Integration of evals accessor method into main client
lib/openai.rb	Added require statement for the new evals module
spec/openai/client/evals_spec.rb	Complete test suite covering all evals, runs, and output_items operations
README.md	Added comprehensive documentation section with 11 usage examples
spec/fixtures/cassettes/*.yml	38 VCR cassette files for test fixtures

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