Add a ListJoiner Component to Merge Multiple Lists into a Single List

[Amnah199]

Is your feature request related to a problem? Please describe.

There is a need for a ListJoiner component in Haystack to handle scenarios where multiple lists (e.g., List[ChatMessage]) need to be merged into a single flat list. This component would simplify workflows by consolidating variadic inputs into one unified list, eliminating nested structures. The output order would respect the pipeline's execution sequence, with user-provided inputs always added first.
Current joiners cannot provide this functionality.

Describe the solution you'd like

For reference, a similar implementation exists in the Haystack cookbook
The above reference also describes one of the use cases of ListJoiner.

Describe alternatives you've considered
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Additional context
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