Feat: Support running AIBrix in local mode

[Jeffwan]

Pull Request Description

Support local mode - easier to debug and connect to engine for testing purpose.

Related Issues

Resolves: #1951 #2040

not all contributors have cluster environment, they can use such ways to test against aibrix router & kv cache or even benchmark later.
This PR is to unblock their development.

Important: Before submitting, please complete the description above and review the checklist below.

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Contribution Guidelines (Expand for Details)

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Pull Request Title Format

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• [Bug]: Corrections to existing functionality
• [CI]: Changes to build process or CI pipeline
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• PR title includes appropriate prefix(es)
• Changes are clearly explained in the PR description
• New and existing tests pass successfully
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• Documentation updated to reflect changes (if applicable)
• Thorough testing completed, no regressions introduced

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the developer experience by introducing a local mode for AIBrix. This new capability allows engineers to run the core gateway components directly on their machines, bypassing the need for containerization or orchestration tools like Kubernetes. This change streamlines the debugging process, accelerates local testing cycles, and provides a more accessible environment for validating gateway functionality, ultimately improving development efficiency.

Highlights

• Local Mode Support: Introduced a new 'local mode' for AIBrix, allowing the gateway (Envoy + gateway-plugin) to run as bare processes without Docker or Kubernetes.
• Simplified Development & Debugging: This mode is designed to facilitate local development, debugging of routing algorithms, single-node testing, and quick validation of gateway behavior.
• Direct vLLM Engine Discovery: The local setup uses static configuration to directly discover and route requests to vLLM engine instances.
• Comprehensive Documentation & Scripts: Provided detailed README.md documentation, example configuration files (endpoints.yaml, envoy.yaml), and convenience scripts (run-local.sh, stop-local.sh) for easy setup and management of the local environment.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable local mode for running AIBrix, which will greatly simplify development and testing. The implementation is well-structured, with clear documentation, configuration files, and helper scripts. My review focuses on improving the user experience by ensuring consistency in the documentation and increasing the robustness of the startup script. I've suggested aligning the example model names across all files to prevent user errors and replacing a fixed sleep with a more reliable polling mechanism in the run-local.sh script.