internal: add datacatalog client
internal: add datacatalog client

Change-Id: Ie32f0c9fba17459a4a79c656a95a0ce47cb1d96a
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/53910
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>
Reviewed-by: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>

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