Add a new Locust-based perf workload under workloads/query-execution/change-streams to benchmark the $_internalSearchIdLookup/ idLookup path. Given that internal search idLookup stage now builds on the same single-document lookup executor abstraction, this makes idLookup a good candidate for a dedicated perf workload under the existing change streams workload suite.

The goal is to establish a baseline for idLookup performance and make regressions easy to detect over time.