ci: add automated PR benchmark comparison with performance alerts

[Copilot]

Implements automated performance tracking for pull requests to surface regressions before merge.

Changes

• Benchmark execution: Modified to output JSON format for structured comparison

  ./casbin_benchmark --benchmark_format=json --benchmark_out=benchmark_result.json

• GitHub Action integration: Added benchmark-action/github-action-benchmark@v1

  • Compares PR benchmarks against master baseline
  • Posts automated PR comments with performance deltas
  • Stores results on master for historical tracking
  • Alert threshold: 150% (flags >50% performance degradation)
  • Non-blocking: warns but doesn't fail CI

• Permissions: Added contents: write and pull-requests: write for result storage and PR comments

Behavior

On PRs: Runs benchmarks, compares to master baseline, posts comment with performance comparison

On master: Runs benchmarks and persists results for future PR comparisons

Original prompt

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This section details on the original issue you should resolve

<issue_title>ci: automated PR performance benchmark comparison</issue_title>
<issue_description>We should implement a CI workflow to execute benchmarks on each PR:

• show performance delta between the new changes and the main branch.

• use automated comments to display benchmark results directly.

• ensure new features do not slow down the library.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ci: automated PR performance benchmark comparison #266

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