Custom endpoint support

[baramofme]

Custom endpoint support needed.

For example

• Connecting to a proxy server that automatically detects API KEY EXAUTED and replaces the KEY.
• Connecting to an LLM gateway that detects QUATA and provides its own rate limit.

[kiddos]

I'm not sure rather I am going to support this. For personal use, I never ran out of free quota on my own before.