Autocomplete: add StarCoder2 hybrid feature flag

[valerybugakov]

Context

• Preparing for the StarCoder2 A/B test.
• Adds starcoder2-hybrid model to the Fireworks provider, which uses starcoder2-7b and starcoder2-15b.
• Cody-Gateway support Cody Gateway: add StarCoder2 models sourcegraph#61207

Test plan

• Tested locally by mocking the return value of the feature flag provider and asserting that the right requests were made by investigating the output console.