langchain[patch]: Extract _aperform_agent_action from _aiter_next_step from AgentExecutor (#15707)
- **Description:** extreact the _aperform_agent_action in the
AgentExecutor class to allow for easier overriding. Extracted logic from
_iter_next_step into a new method _perform_agent_action for consistency
and easier overriding.
- **Issue:** #15706
Closes #15706