Contribute to Aider to set user agent

[jhrozek]

Since codegate would benefit to know which coding assistant is talking to it, we should enhance aider to send either a user-agent or a user-agent-like header.

[jhrozek]

It turned out to be non-trivial to add the traditional user-agent header to aider because it uses litellm which in turn uses the openAI python library where the only way to add a user-agent seems to be to override the httpx.Transport. Because this might be complicated to pass through down the stack (aider -> litellm -> openai) and might have side-effects, I opted to add a custom header to ader instead in Aider-AI/aider#2988