Refactor agents and tools (#91)

* Move tools to agents

* Move agents to dedicate place

* Remove subclassing BaseAgent from BaseTool

knowledgehub/agents/utils.py

@@ -0,0 +1,22 @@
+from kotaemon.base import Document
+
+
+def get_plugin_response_content(output) -> str:
+    """
+    Wrapper for AgentOutput content return
+    """
+    if isinstance(output, Document):
+        return output.text
+    else:
+        return str(output)
+
+
+def calculate_cost(model_name: str, prompt_token: int, completion_token: int) -> float:
+    """
+    Calculate the cost of a prompt and completion.
+
+    Returns:
+        float: Cost of the provided model name with provided token information
+    """
+    # TODO: to be implemented
+    return 0.0