Relates tocoder/internal#911

This PR adds a scaletest runner that simulates a user with a workspace configured to autostart at a specific time.

An instance of this autostart runner does the following:

1. Creates a user.
2. Creates a workspace.
3. Listens on/api/v2/workspaces/<ws-id>/watch to wait for build completions throughout the run.
4. Stops the workspace, waits for the build to finish.
5. Waits for all other concurrently executing runners (per theSetupBarrierWaitGroup) to also have a stopped workspace.
6. Sets the workspace autostart time totime.Now().Add(cfg.AutoStartDelay)
7. Waits for the autostart workspace build to begin, enter thepending status, and thenrunning.
8. Waits for the autostart workspace build to end.

Exposes four prometheus metrics:

• autostart_job_creation_latency_seconds - HistogramVec. Labels = {username, workspace_name}
• autostart_job_acquired_latency_seconds - HistogramVec. Labels = {username, workspace_name}
• autostart_total_latency_seconds -HistogramVec. Labels ={username, workspace_name}.
• autostart_errors_total -CounterVec. Labels ={username, action}.