Python: basic python a2a support (#906)

* basic python a2a support

* fixes

* small fixes

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

python/packages/main/agent_framework/a2a/__init__.py

@@ -0,0 +1,24 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+import importlib
+from typing import Any
+
+PACKAGE_NAME = "agent_framework_a2a"
+PACKAGE_EXTRA = "a2a"
+_IMPORTS = ["__version__", "A2AAgent"]
+
+
+def __getattr__(name: str) -> Any:
+    if name in _IMPORTS:
+        try:
+            return getattr(importlib.import_module(PACKAGE_NAME), name)
+        except ModuleNotFoundError as exc:
+            raise ModuleNotFoundError(
+                f"The '{PACKAGE_EXTRA}' extra is not installed, "
+                f"please do `pip install agent-framework[{PACKAGE_EXTRA}]`"
+            ) from exc
+    raise AttributeError(f"Module {PACKAGE_NAME} has no attribute {name}.")
+
+
+def __dir__() -> list[str]:
+    return _IMPORTS

python/packages/main/agent_framework/a2a/__init__.pyi

@@ -0,0 +1,5 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+from agent_framework_a2a import A2AAgent, __version__
+
+__all__ = ["A2AAgent", "__version__"]