bigquery: add Client.Close
bigquery: add Client.Close

It is a no-op now but won't be when we are top of gRPC.

Change-Id: If7d41551c9d58779de76575a3b515f618362760e
Reviewed-on: https://code-review.googlesource.com/7613
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go