haystack feat: Trace pipeline run input/output data

Why:

This change is motivated by the need to improve the pipeline's debugging and tracing capabilities by providing more detailed information about the input and output data. Most importantly this change will help us generate better pipeline level traces in langfuse and other integrations.

What:

The specific changes made include:

Adding two new tracing tags, haystack.pipeline.input_data and haystack.pipeline.output_data, to provide more detailed information about the input and output data.
Moving the declaration of final_outputs to the top of the tracing context manager with block, ensuring it is initialized before being used.

How can it be used:

The added tracing tags can be utilized to monitor and analyze the pipeline's input and output data, enabling developers to:

Use the tracing data to debug and optimize the pipeline's performance.
Without these traces we can't capture pipeline inputs/outputs and langfuse integration for pipeline run looks like this:

pipeline_run_before

After these changes langfuse integration for pipeline run looks like this:

pipeline_run_after

How did you test it:

No tests were added, CI run hopefully discovers potential issues

Notes for the reviewer:

Please review the placement of the final_outputs declaration and ensure it does not affect the pipeline's behavior. Additionally, consider adding more tracing tags to provide further insight into the pipeline's operation.