This commit adds support for autoscaling in the InstanceAdminClient with
associated unit tests and a happy-path integration test.

When creating either a new instance or cluster, callers may specify an
AutoscalingConfig.

For existing clusters, to enable autoscaling on a cluster, a caller may
use SetAutoscaling. To remove autoscaling, callers may use
UpdateCluster. Alternatively, callers may either update the cluster to
enable or disable autoscaling with UpdateInstanceWithClusters or
UpdateInstanceAndSyncClusters.

Previously UpdateInstanceWithClusters would try to update a cluster even
if the NumNodes was 0 (or less), resulting in an erroneous error. This
commit ensures a cluster is updated only when a valid and non-zero value
of NumNodes is provided.