roachtest/mixedversion: add an option to execute a user-specified plan

Currently, the mixedversion framework uses `testPlanner.Plan()` to generate a randomized `TestPlan`. The `seed` alone no longer suffices to reproduce the `TestPlan`. We also need `testOptions`, (e.g., `testOptions.maxNumPlanSteps` could require multiple invocations of `testPlanner.Plan()`), and the commit hash. Thus, when debugging an existing test plan on a different branch, it's not easy to reproduce it. There are also situations where a test plan could be either constructed out-of-band or generated via `MVT_DRY_RUN`.

We already pretty-print the plan. However, it's not really a well-structured format; e.g., `debug` flag alters some of the output. Serializing the whole plan in json/yaml would obviate the need for a custom parser.

Jira issue: CRDB-52166