Autocomplete: add cody.autocomplete.experimental.fireworksOptions

[valerybugakov]

Context

• Adds an experimental setting for internal use, cody.autocomplete.experimental.fireworksOptions for quick local experimentation with new models via direct access to the Fireworks API. This PR makes it easy for anyone to experiment with new models without hacking on autocomplete internals locally.
• This does not use cody.autocomplete.advanced.model because it lists supported models with validation. The fireworksOptions.model is supposed to be used on new models for internal testing.

Test plan

1. Add the following settings:

  "cody.autocomplete.experimental.fireworksOptions": {
    "url": "https://api.fireworks.ai/inference/v1/completions",
    "token": "YOUR_ACCESS_TOKEN",
    "model": "accounts/sourcegraph/models/starcoder2-7b",
    "parameters": {
      "stop": [
          "<fim_prefix>",
          "<fim_suffix>",
          "<fim_middle>",
          "<|endoftext|>",
          "<file_sep>",
      ]
    }
  },

2. Autocomplete should work as usual.