Replace usage of Generators with ChatGenerators in all tutorials

[julian-risch]

We should replace the usage of Generators with ChatGenerators in all tutorials so that users are prepared to use ChatGenerators in future and benefit from all future new features of ChatGenerators such as function calling, async, etc.

Tasks

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1. 27_First_RAG_Pipeline. Code changes are drafted in this notebook.
2. 28_Structured_Output_With_Loop
3. 29_Serializing_Pipelines
4. 30_File_Type_Preprocessing_Index_Pipeline
5. 32_Classifying_Documents_and_Queries_by_Language
6. 35_Evaluating_RAG_Pipelines
7. 36_Building_Fallbacks_with_Conditional_Routing. Code changes are drafted in this notebook.
8. 37_Simplifying_Pipeline_Inputs_with_Multiplexer. We should simply remove this tutorial because the Multiplexer component was removed in 2.4.0.
9. 40_Building_Chat_Application_with_Function_Calling
10. I suggest to create a tutorial or at least a cookbook for the Self-Reflecting Agents with Loops DLAI course lesson based on the work that has already been done.

Related issues:

• Create a version of tutorial "First QA Pipeline with RAG" using ChatGenerator #349
• Create a version of DLAI lesson "Self-Reflecting Agents with Loops" (entity extraction) using ChatGenerator haystack#8410
• Create a version of tutorial "Building Fallbacks to Websearch with Conditional Routing" using ChatGenerator #350

[julian-risch]

Didn't work on this so I'll move it back to backlog.

[julian-risch]

Moving this issue back to backlog because of lack of time to work on it. We should also first merge the open PRs with ChatMessage changes.