feat: add LightLLM provider support

[kelvinguo0210]

With this change, you can specify a LightLLM provider for Codex.

Command line:

export LIGHTLLM_API_KEY=
export LIGHTLLM_BASE_URL=http://xxx.xxx.xxx.xxx:port

node ./codex-cli/dist/cli.js --provider lightllm --model claude-3-7-sonnet --fullAuto
or:
codex --provider lightllm --model claude-3-7-sonnet --fullAuto

config.yaml
The example LightLLM configuration file:
model_list:

• model_name: claude-3-7-sonnet
  litellm_params:
  model: bedrock/us.anthropic.claude-3-7-sonnet-20250219-v1:0
  aws_access_key_id:
  aws_secret_access_key:
  aws_region_name: us-east-1
  api_key:

LightLLM Deploy:
docker run -d -v /tmp/litellm/config.yaml:/app/config/config.yaml -p 9080:4000 ghcr.io/berriai/litellm:main-latest --config /app/config/config.yaml --detailed_debug

Test:

• codex can run against lightLLM with claude-3-7-sonnet as backend model normally.

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[kelvinguo0210]

I have read the CLA Document and I hereby sign the CLA