Switch CodSpeed benchmarks to OIDC authentication and add badge

[avalanche-staging]

Summary

This PR migrates the CodSpeed benchmarks workflow from token-based authentication to OpenID Connect (OIDC) authentication and adds a CodSpeed performance badge to the README.

Changes

Workflow (.github/workflows/benchmarks.yml):

• Added id-token: write permission to enable OIDC authentication with CodSpeed, replacing the previous secrets.CODSPEED_TOKEN approach. OIDC is more secure and doesn't require managing repository secrets.
• Added workflow_dispatch trigger to allow CodSpeed to trigger backtest performance analysis for generating initial baseline data.
• Removed the token parameter from the CodSpeed action step (no longer needed with OIDC).

README (README.md):

• Added the CodSpeed performance badge alongside existing project badges.

Next steps

• Ensure the CodSpeed GitHub App is installed on this repository for OIDC authentication to work.
• After merging, CodSpeed can use workflow_dispatch to trigger backtests and build initial performance baselines.

[avalanche-staging]

Congrats! CodSpeed is installed 🎉

🆕 142 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• wc_chars_large_line_count[100000]: 1 ms
• wc_bytes_synthetic[500]: 190.8 µs
• wc_words_large_line_count[100000]: 48 ms
• wc_lines_large_line_count[500000]: 2.9 ms
• wc_lines_variable_length[(50, 500)]: 3.4 ms
• wc_words_synthetic[2000]: 11.4 s
• wc_default_large_line_count[100000]: 50.4 ms
• wc_lines_extreme_line_lengths[(100000, 200)]: 1.6 ms
• wc_bytes_lines_synthetic[2000]: 120.2 ms
• cksum_blake2b: 183.6 ms
• cksum_bsd: 128.3 ms
• cksum_sysv: 48.2 ms
• cksum_shake256: 1.9 s
• cksum_sha256: 1.8 s
• cksum_blake3: 212.3 µs
• cksum_shake128: 1.5 s
• cksum_sha384: 1.1 s
• cksum_crc: 17 ms
• cksum_sha3: 1.8 s
• cksum_crc32b: 13.9 ms
• ...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

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[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/cp-mv-enotsup-xattr. tests/cp/cp-mv-enotsup-xattr is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/csplit/csplit-io-err. tests/csplit/csplit-io-err is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)