[model-gateway] fix version output

[slin1237]

1. fix version output to be sgl-model-gateway instead of sgl-router
2. update version to read from cargo instead of python binding
3. use macro for env vars across version and build
4. remove printlns
5. chang env var from SGL_ROUTER_* to SGL_MODEL_GATEWAY_*

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

[gemini-code-assist]

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