Add etc/system_perf_soak.yml, the Evergreen project config for the sys-perf-soak project (System Performance Soak Tests). Unlike etc/system_perf.yml, it deliberately includes only base.yml, compiles.yml, shared_tasks.yml, and the new soak_variants.yml from the DSI module — not variants.yml/master_variants.yml — so the soak project exposes only the long-running soak build variants and users are not made to browse the ~180 unrelated sys-perf variants.

This is the mongo-repo companion to DEVPROD-32152, which isolates the multi-day soak test variants into a dedicated Evergreen project so their timeouts are not capped by sys-perf's global MaxExecTimeoutSecs.

The file is inert until the sys-perf-soak project's repotracker is enabled: the project (already created) points at this path, but no mainline versions are generated yet. The sys-perf project is unaffected — nothing in etc/system_perf.yml references this file.

Additional Context

• DEVPROD-32152 Add soak tests and isolate their variants to sys-perf-soak{_}: the DSI-side change defining the soak variants/tasks this config includes.
• SERVER-130585 Include compiles_pgo.yml in the sys-perf Evergreen config_: earlier mongo-side companion in the same effort (compiles split that made compiles.yml includable standalone).