populate X-Server-Timeout when `timeout` is set in `jobs.insert` requests

[tswast]

https://cloud.google.com/apis/docs/system-parameters#definitions

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

While investigating internal issue 195337762, it was determined that some internal servers can get stuck waiting for longer than is necessary. It is believed that if X-Server-Timeout is set, then those internal servers can stop processing more quickly.

Describe the solution you'd like

Whenever timeout is set in the JSONConnection call API method, also populate the X-Server-Timeout header.

Describe alternatives you've considered

It's possible the local client-side timeout may be sufficient.

Additional context

It's still not 100% clear to me that this header will help. A Googler on the internal issue 195337762 promises to share some docs on how this header bubbles through the system.

[tswast]

After offline discussion, I'm moving this back to the BigQuery repo. I'm not comfortable translating timeout to X-Server-Timeout in all APIs affected by google-cloud-core, but after some testing it appears that the server does the right thing for the BigQuery jobs.insert APIs, so let's add the header for those.