ENH: Add code_execute tool to unify CodeAgent & ToolCallingAgent (#1956)

[janhaslik]

Add support for executing code by a tool call for ToolCallingAgent.
Python code executed via code_execute has access to all tools as callable functions.

Issue: #1956

Changes:

toolcalling_agent.yml:

• Add section that mentions the existence of a the code_execute tool and how to use it (tool calls as regular function calls)
• Simple example

agents.py:

• _execute_code function for executing the python code uses PythonExecutor
• optional class params for setting and initializing the relevant parameters
• Update to execute_tool_call function to handle the code_execute tool

test_agents.py:

• test_toolcalling_agent_calls_code_execute (Test that the ToolCallingAgent can execute simple Python code using code_execute)
• test_toolcalling_agent_calls_tool_from_tools_list (Test that Python code executed via code_execute can call tools from the agent's tool list)