initial hacking towards an OutputHandler

[kafonek]

cargo run should print out the string add_cell_content: {"text/plain": String("4")} in addition to the other output from Debug Handler and Message Count Handler.

• Is bubbling up a dictionary of {mimetype: content} what we want for adding to cell content?
• Python version of this basically did the same thing, pushing up content -- where content.data was the dictionary of mimetype / raw content https://github.com/kafonek/kernel-sidecar/blob/main/src/kernel_sidecar/handlers/output.py#L41

[kafonek]

Current dependencies on/for this PR:

• main
  • PR refactor handlers to directory structure #27
    • PR initial hacking towards an OutputHandler #28 👈
      • PR Model clear_output message #29
        • PR Add clear output to OutputHandler #30

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