Add CodSpeed continuous performance testing integration

[avalanche-staging]

Summary

This PR integrates CodSpeed for continuous performance testing of the scipp C++ codebase. CodSpeed provides accurate, reproducible performance measurements in CI environments using specialized instrumentation to isolate performance from environmental noise.

Changes Made

• CMake Integration: Modified lib/CMakeLists.txt to use CodSpeed's google_benchmark compatibility layer via FetchContent when CODSPEED_MODE is set
• CI Workflow: Created .github/workflows/codspeed.yml to run benchmarks on push to main, pull requests, and manual triggers
• Documentation: Added CodSpeed badge to README.md linking to the project's performance dashboard

Technical Details

• Uses CodSpeed's simulation mode for consistent, hardware-agnostic measurements
• Leverages existing Google Benchmark infrastructure with zero code changes to benchmark files
• Authenticates via OIDC (OpenID Connect) for secure, token-free uploads
• Builds all benchmarks using the existing all-benchmarks CMake target
• Runs on Ubuntu 24.04 with the same conda environment as the main CI

Next Steps

Once this PR is merged, you'll be able to:

• View performance trends over time on the CodSpeed dashboard
• Receive automated performance regression detection on pull requests
• See detailed flame graphs and execution profiles for benchmarks

The first workflow run will establish baseline performance metrics for future comparisons.