Feature proposal: Add --profile for conditional config

[jdanbrown]

See this issue for details:

• https://github.com/Aider-AI/aider/issues/4602

Remaining work before merging:

• [ ] Add unit tests
  • Override precedence: cli > env > profile > config
  • Multiple --profile=x are allowed, keys from later ones override earlier ones
• [ ] Update docs
• [ ] Is this implementation well organized?
  • Not sure if I'm messing up intended module/function boundaries.

I'd be happy to complete all these TODOs if maintainers give me the go ahead!

[CLAassistant]

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