Support custom prices for models

[donfour]

I have heard at least 3 users ask for it

Issues:

• Not all users can do a redeploy whenever there's a new model or price change
• Some users want to calculate prices for open-source models
• Some users might not know of a price change until some time after

Possible features:

• Allow users to override default input and output prices, or add a new model (e.g. gpt-4o), for commercial LLM providers
• Allow users to set custom input and output prices for open-source LLM providers
• Allow users to update past event prices retroactively