feat: adds line item example for cost API

[thli-openai]

Summary

This PR addresses a few items:

1. Refactors the usage / cost API calls and pagination into its own helper function to reduce duplicate code
2. Corrects the total cost example in the Cost API which was incorrectly labeled as line item costs
3. Adds additional example for a line item cost stacked bar chart

Motivation

Why are these changes necessary? How do they improve the cookbook?

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[MW-OAI]

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[MW-OAI]

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