Adding create_preference_id client method.

[acompa]

Instead of demonstrating how users can create preference ids, let's do it for them.

This PR introduces a request method create_preference_id shamelessly cribbed from our docs. It's available both as a standalone method and a instance method of NotDiamond.

[acompa]

Unit tests once I have a bit more time.

[acompa]

Okay, test ready - passes locally for me as well.

Looks like the builds previously failed with 529s - probably some concurrent API key usage for providers.

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
notdiamond/llms
client.py
request.py					308
notdiamond/toolkit
custom_router.py
tests/test_components/test_llms
test_llm.py
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