Add `.prompt_template()` to all LLM-related components in sdialog

The same as all LLM-based components have a `.prompt()` we should have `.prompt_template()` to get the prompt template too.

**Idia:** make this function as `prompt_template(self, **kwargs)` and if kwargs are given they're going to be used to render the template. Then we can use internally this function to render the templates and we kill two bird with one stone :)