.Python: Added Agent and AgentThread abstractions (#130)

* Added Agent and AgentThread classes

* Addressed PR feedback

* Converted Agent to protocol

* Removed thread deletion logic

* Small update

* Small updates to the Agent protocol

python/agent_framework/exceptions.py

@@ -5,3 +5,15 @@ class AgentFrameworkException(Exception):
     """Base class for exceptions in the Agent Framework."""
 
     pass
+
+
+class AgentException(AgentFrameworkException):
+    """Base class for all agent exceptions."""
+
+    pass
+
+
+class AgentExecutionException(AgentException):
+    """An error occurred while executing the agent."""
+
+    pass