feat: progress bar during query in `QueryJob.to_dataframe` and `QueryJob.to_arrow`

[tswast]

Is your feature request related to a problem? Please describe.

When running a query via the %%bigquery magics or waiting for it to finish via QueryJob.to_dataframe or QueryJob.to_arrow, an argument progress_bar_type is accepted.

Currently, this only shows the progress of the query results download. It would be great if it would also give an indicator while the query is executing.

Describe the solution you'd like

When a value is passed to progress_bar_type, show some kind of progress bar. Ideally, it would work similarly to the UI. For example,

• Show the job state in the progress bar description. For example, is it currently "pending" (queued, waiting for resources) or "running" (actually executing).
• Show how many "stages" there are via length of the query_plan.
• Find the latest incomplete stage.
• Use parallel_inputs as the total amount of work (per stage)
• Use completed_parallel_inputs as the amount of work completed so far.

To populate this, instead of calling result() once:

• Call result(timeout=[a few seconds]) every few seconds.
• Call job.reload() to fetch the latest job statistics.
• Update the progress bar.
• Repeat.

Describe alternatives you've considered

• Some kind of spinner that only shows that some time has elapsed. (This is actually harder than it sounds because tqdm doesn't actually support spinners, and if it ever does, it sounds like it'll be via a different API than the current progress bars. "Indefinite" progress bar tqdm/tqdm#427 Add spinners tqdm/tqdm#925

Additional context

• pandas-gbq feature request: Implement initial "waiting" logs with tqdm? python-bigquery-pandas#327

• Example of BigQuery UI progress bar:

  

[tswast]

Some additional thoughts: with dynamic query planning, we don't actually know how many stages will be required, so using these stats as a progress bar could be misleading.

It'd still be useful to show that the query is making progress somehow.

[tswast]

If we do show stage progress, we'll want to periodically update the description with Stage 1/X and elapsed seconds like the UI does.

https://github.com/tqdm/tqdm/blob/0f823e79f303b4a93ef1381badb1e65757e5070f/tqdm/std.py#L1413