feat: Add OpenAI Agents SDK integration service

- Create new OpenAIAgentService that integrates OpenAI Agents SDK with Pipecat
- Support for agent loops, handoffs, guardrails, and session management
- Add streaming and non-streaming response modes
- Include comprehensive tool integration and error handling
- Add optional dependency for openai-agents package
- Create foundational examples showing basic usage and agent handoffs
- Add comprehensive tests with mocked dependencies
- Include detailed documentation and README

Key features:
- Real-time streaming responses compatible with Pipecat pipelines
- Agent handoffs for specialized task delegation
- Tool calling with automatic schema generation
- Input/output guardrails for safety and validation
- Session context management for conversation continuity
- Built-in tracing and monitoring integration

Examples:
- 45-openai-agent-basic.py: Basic agent with weather and trivia tools
- 46-openai-agent-handoffs.py: Multi-agent system with specialist handoffs

pyproject.toml

@@ -83,6 +83,7 @@ nim = []
 neuphonic = [ "websockets>=13.1,<15.0" ]
 noisereduce = [ "noisereduce~=3.0.3" ]
 openai = [ "websockets>=13.1,<15.0" ]
+openai-agent = [ "openai-agents~=1.0.0" ]
 openpipe = [ "openpipe~=4.50.0" ]
 openrouter = []
 perplexity = []