Core: Add timeout parameter to connection's api_request() method #9914

As a client library developer, I want to have an easy way of specifying the timeout at the transport level for HTTP requests.

Motivation:
If the transport layer makes an HTTP request that gets stalled, it can block the client. Any timeouts and retries on top of it cannot come into play, because the transport layer blocks them while waiting for the response without any timeout specified.

One of the manifestations of the problem in the BigQuery client: #7831