Support custom Whisper API endpoints for voice transcription

[mbailey]

Issue

It was only on reading CONTRIBUTING.md today that I realised I should have:

• Created an issue before working on this
• Used pre-commit

Sorry to foist a largish change without creating an issue first @paul-gauthier.

I think this is a very important change as Whisper is free and people can self host it to protect their privacy and money. I've been using Aider with self hosted Whisper for months using a simple proxy I wrote but thought I would make the effort to make it easy for others. It's been a good chance to get to know Aider better.

I rewrote #2634 tonight to use --api-key and renamed the env vars to WHISPER_API_BASE and WHISPER_API_KEY.

Anyway, I think the change is now pretty tight and minimal.

Version and model info

No response