Define targets in codspeed.yaml to execute with codspeed run

[GuillaumeLagrange]

1. Add an stdin mode for the exec-harness, which allows running multiple commands. Commands are passed as JSON through stdin when invoking exec harness with exec-harness -
2. Fix behavior of exec harness in walltime when executing different benchmarks
3. Parse targets from codspeed.yaml when running codspeed run without arguments, and pass them to the exec-harness

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

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_{Comparing cod-1906-define-exec-targets-in-codspeedyaml (08878c1) with main (8a39cf5)}