Reasoning support in `StreamingChunk`

[anakin87]

When working on deepset-ai/haystack-core-integrations#2200, I realized that support for streaming reasoning text in Haystack is currently limited.
In that PR, reasoning chunks are only accumulated in StreamingChunk.meta for later reconstruction, but they are not actually streamed.

What needs to be done

• introduce reasoning field in StreamingChunk to store reasoning chunks
  • this might be a str.
  • investigate if a more complex object is needed by looking at how providers stream reasoning chunks (Anthropic)
• update print_streaming_chunk utility function to also print reasoning