examples : Rewrite pydantic_models_to_grammar_examples.py (#8493)

Changes:

- Move each example into its own function. This makes the code much
  easier to read and understand.
- Make the program easy to only run one test by commenting out function
  calls in main().
- Make the output easy to parse by indenting the output for each example.
- Add shebang and +x bit to make it clear it's an executable.
- Make the host configurable via --host with a default 127.0.0.1:8080.
- Make the code look in the tools list to call the registered tool,
  instead of hardcoding the returned values. This makes the code more
  copy-pastable.
- Add error checking, so that the program exits 1 if the LLM didn't
  returned expected values. It's super useful to check for correctness.

Testing:

- Tested with Mistral-7B-Instruct-v0.3 in F16 and Q5_K_M and
  Meta-Llama-3-8B-Instruct in F16 and Q5_K_M.
  - I did not observe a failure even once in Mistral-7B-Instruct-v0.3.
  - Llama-3 failed about a third of the time in example_concurrent: it
    only returned one call instead of 3. Even for F16.

Potential follow ups:

- Do not fix the prompt encoding yet. Surprisingly it mostly works even
  if the prompt encoding is not model optimized.
- Add chained answer and response.

Test only change.

Author: maruel