bigquery: export ProjectID on DatasetIterator
bigquery: export ProjectID on DatasetIterator

Allow users to set the project ID of a DatasetIterator by assigning to a
field before use, as we do for other properties of the iterator.

Deprecate the DatasetsInProject method.

Can anyone think of a reason we didn't do it this way in the first place?
It seems simpler and more consistent.

Also, document that the iterator's exported fields should be set initially.

Change-Id: Ic72cc88c125599a20e085caa0c6f6296eb31f21d
Reviewed-on: https://code-review.googlesource.com/16670
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Sai Cheemalapati <36dc41adb0ea108898de35222b4f3585eb1d8968@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go