default (per API request) timeout of 5 minutes #889

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

Since we don't have a timeout by default on our HTTP calls, it's possible that they hang much longer than expected.

Describe the solution you'd like

Set a default timeout to 5 minutes.

As I found in #859, many of the backend BigQuery APIs have a deadline of 4 minutes. Giving ourselves 5 minutes per request should be enough time to account for any network latency and download time.

Describe alternatives you've considered

Status quo. -- This makes the time that a request can complete less predicable though.

Additional context

I recall we tried this once before, but reverted it due to too many flakey tests: googleapis/google-cloud-python#10219

That was with 60 seconds, whereas the backend deadline is 4 minutes, so perhaps it's not all too surprising that there were flakes?