Isolate template generation to override

[gael-ft]

Hello,

Little PR to allow applications to easily add variables in jinja prompt templates.

Currenly, if we look at the planning for example, the method is quite big.
But at the same time, I think adding/updating templates and their variables is a very common usecase.

from smolagents import CodeAgent

class MyAgent(CodeAgent):
  # ...
  
  # Assuming I added a jinja block using {{client_info}}
  @override
  def _populate_planning_initial_plan_template(self, task: str, step: int) -> str:
    variables = {"client_info": self.client_info, "task": task, "tools": self.tools, "managed_agents": self.managed_agents}
    return populate_template(
            self.prompt_templates["planning"]["initial_plan"],
            variables=variables,
        )