self correction loop for json parser

[Timoeller]

Enable deepset-ai/haystack-demos#23 within haystack natively.
In the PR description there are 2 todos left open to fully enable this feature in haystack:

1. We need optional variables in PromptTemplates (we are using a haystack branch so far)
2. Allow three variables in the prompt template so that we can include the error message (so far only the wrongly generated output is used)

Additional context
This feature is important to showcase haystack 2.0 new feature around loops within pipelines

[tholor]

We need optional variables in PromptTemplates (we are using a haystack branch so far)

To add some context here: In the hackathon for the demo, we simply
changed this line to
component.set_input_types(self, **{var: Optional[Any] for var in template_variables}) to enable one optional input (replies). However, this somehow didn't work with a second optional input we wanted (error_msg).

[tholor]

blocked by #6356