sql: randomized performance test for plan_cache_mode=auto

In order to find cases where `plan_cache_mode=auto` results in generic query plans which perform worse than their corresponding custom query plans, we could performed randomized tests that:

1. Generate a random schema and data.
2. Generate a random queries.
3. For each query:
a. Run it 6 times.
b. If a generic plan is chosen on the 6th execution, fail the test if it is significantly slower than the first 5 executions.

For 3b, comparing query latency might lead to flaky tests. Instead, we could compare the total number of rows decoded from KV, which should be more deterministic.

Jira issue: CRDB-57214