Support for PortKey API gateway or extra_headers env variable

[jglamine]

Issue

I'm trying to connect to PortKey, which is an LLM API gateway. Portkey provides an OpenAPI compatible API, but it requires an extra x-portkey-virtual-key header to route to the appropriate underlying model/configuration.

LiteLLM supports passing extra_headers. Could we expose this to Aider?

Possible solutions

1. Add direct support for PortKey with a PORTKEY_VIRTUAL_KEY env variable.
2. Add support for an OPENAI_API_EXTRA_HEADERS env variable which contains a list of extra headers to pass.

Option 2 seems more flexible.

Version and model info

None