[Feature]: Fallback Prompts - model-specific

[krrishdholakia]

The Feature

Allow user to specify model specific prompts if fallbacks are triggered for a certain requests

e.g.

{
  "metadata": {
    "fallback": {
      "claude-3-opus": "Opus-specific system message",
      "claude-3-sonnet": "Sonnet-specific system message"
      ...
     }
  }
}

Motivation, pitch

User would rather fail requests currently, than use fallbacks and get back bad answers

Twitter / LinkedIn details

cc: @dkindlund