Model usage tracking and monthly consumption limit

[br1-s]

This pull request introduces new features to enhance the usage tracking of LLM (Language Model) per user and provides an option to set a monthly consumption limit.

The following changes have been made:

• Added two MongoDB collections, userLlmUsage and llmPriceRate, to store user-specific LLM usage and the current prices of LLM respectively.
• Implemented utility methods to save and validate user LLM usage.
• Updated the OpenAIModelSchema to include the model type (chat, completion, embedding).
• Configured the initial database settings for LLM pricing and updated the readme file with detailed instructions.
• Added a new environment variable DEFAULT_USER_LIMIT_USD_MONTHLY, which allows configuring a default monthly limit for each user.

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