feat: Add A2A (Agent-to-Agent) protocol support for remote interoperability

- Implement CrewAgentExecutor class that wraps CrewAI crews as A2A-compatible agents
- Add server utilities for starting A2A servers with crews
- Include comprehensive test coverage for all A2A functionality
- Add optional dependency group 'a2a' in pyproject.toml
- Expose A2A classes in main CrewAI module with graceful import handling
- Add documentation and examples for A2A integration
- Support bidirectional agent communication via A2A protocol
- Enable crews to participate in remote agent networks

Fixes #2970

Co-Authored-By: João <joao@crewai.com>

examples/a2a_integration_example.py

@@ -0,0 +1,64 @@
+"""Example: CrewAI A2A Integration
+
+This example demonstrates how to expose a CrewAI crew as an A2A (Agent-to-Agent)
+protocol server for remote interoperability.
+
+Requirements:
+    pip install crewai[a2a]
+"""
+
+from crewai import Agent, Crew, Task
+from crewai.a2a import CrewAgentExecutor, start_a2a_server
+
+
+def main():
+    """Create and start an A2A server with a CrewAI crew."""
+    
+    researcher = Agent(
+        role="Research Analyst",
+        goal="Provide comprehensive research and analysis on any topic",
+        backstory=(
+            "You are an experienced research analyst with expertise in "
+            "gathering, analyzing, and synthesizing information from various sources."
+        ),
+        verbose=True
+    )
+    
+    research_task = Task(
+        description=(
+            "Research and analyze the topic: {query}\n"
+            "Provide a comprehensive overview including:\n"
+            "- Key concepts and definitions\n"
+            "- Current trends and developments\n"
+            "- Important considerations\n"
+            "- Relevant examples or case studies"
+        ),
+        agent=researcher,
+        expected_output="A detailed research report with analysis and insights"
+    )
+    
+    research_crew = Crew(
+        agents=[researcher],
+        tasks=[research_task],
+        verbose=True
+    )
+    
+    executor = CrewAgentExecutor(
+        crew=research_crew,
+        supported_content_types=['text', 'text/plain', 'application/json']
+    )
+    
+    print("Starting A2A server with CrewAI research crew...")
+    print("Server will be available at http://localhost:10001")
+    print("Use the A2A CLI or SDK to interact with the crew remotely")
+    
+    start_a2a_server(
+        executor,
+        host="0.0.0.0",
+        port=10001,
+        transport="starlette"
+    )
+
+
+if __name__ == "__main__":
+    main()