Introduce the ability to cycle between alternative inline assists (#18098)

Release Notes:

- Added a new `assistant.inline_alternatives` setting to configure
additional models that will be used to perform inline assists in
parallel.

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Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Roy <roy@anthropic.com>
Co-authored-by: Adam <wolffiex@anthropic.com>