adding deepseek and lang specific mixtral for completions ab experiment

[sagtanih]

Context

1. Adds support for deepseek-coder model and updated Mixtral finetuned-FIM models identifiers hosted on Fireworks.
2. Client side pull request: https://github.com/sourcegraph/cody/pull/4577

Test plan

curl -vS -X POST http://localhost:9992/v1/completions/fireworks -H 'Authorization: bearer <SGD_TOKEN>' -d '{"stream":false,"max_tokens":50, "model": "accounts/sourcegraph/models/deepseek-coder-7b-base", "stop_sequences": ["\n\n"], "prompt": "const value = ", "stream":false}' -H 'X-sourcegraph-feature: code_completions'

curl -vS -X POST http://localhost:9992/v1/completions/fireworks -H 'Authorization: bearer <SGD_TOKEN>' -d '{"stream":false,"max_tokens":50, "model": "accounts/sourcegraph/models/custom-deepseek-1p3b-base-hf-version", "stop_sequences": ["\n\n"], "prompt": "const value = ", "stream":false}' -H 'X-sourcegraph-feature: code_completions'

curl -vS -X POST http://localhost:9992/v1/completions/fireworks -H 'Authorization: bearer <SGD_TOKEN>' -d '{"stream":false,"max_tokens":50, "model": "fim-lang-specific-model-mixtral", "stop_sequences": ["\n\n"], "prompt": "const value = ", "stream":false}' -H 'X-sourcegraph-feature: code_completions'