Python: [BREAKING] Support code-defined agent skills (#4387)

* support code skills

* address pr review comments

* address package and syntax checks

* address pr review comments

* address pr review comment

* address failed check

* rename agentskill and agetnskillprovider

* move agent skills related assets to _skills.py

* address pr review comments

* address review comments

python/packages/core/agent_framework/__init__.py

@@ -59,7 +59,7 @@ from ._sessions import (
     register_state_type,
 )
 from ._settings import SecretString, load_settings
-from ._skills import FileAgentSkillsProvider
+from ._skills import Skill, SkillResource, SkillsProvider
 from ._telemetry import (
     AGENT_FRAMEWORK_USER_AGENT,
     APP_INFO,
@@ -205,6 +205,9 @@ __all__ = [
     "AgentResponseUpdate",
     "AgentRunInputs",
     "AgentSession",
+    "Skill",
+    "SkillResource",
+    "SkillsProvider",
     "Annotation",
     "BaseAgent",
     "BaseChatClient",
@@ -234,7 +237,6 @@ __all__ = [
     "Executor",
     "FanInEdgeGroup",
     "FanOutEdgeGroup",
-    "FileAgentSkillsProvider",
     "FileCheckpointStorage",
     "FinalT",
     "FinishReason",