feat: add synchronous and asynchronous OpenAI embeddings

[ArjunR00T]

Description

Adds an OpenAI embedding integration with synchronous and asynchronous APIs, token-aware batching, and clear error handling. Closes #6

Changes

• Added OpenAIEmbedder with configurable model, API key, token limits, batch size, overflow behavior, and concurrency.
• Added lazy OpenAI client management with sync/async context managers and close methods.
• Added a default asynchronous aembed() implementation to the Embedder base class.
• Exported OpenAIEmbedder from ragframework.embeddings.
• Added tiktoken to the [openai] optional dependency.
• Added mocked unit tests for configuration, batching, truncation, ordering, sync/async calls, and error handling.
• Updated the examples README and added an [Unreleased] changelog entry.

Type of change

• Bug fix
• New feature / integration
• Documentation update
• Refactor / code quality

Checklist

• pytest tests/ -v passes locally — collection currently errors in tests/test_document/test_html.py with KeyError: 'file' during pytest parameter-ID generation.
• ruff check ragframework/ passes
• mypy ragframework/ passes
• New or updated tests cover the changes
• Optional dependencies are guarded with a helpful import error
• Docstrings updated where applicable
• CHANGELOG.md updated under [Unreleased]