feat: Generate client for datacatalog

[tswast]

Verified manually that the client works with:

from google.cloud import datacatalog_v1beta1

client = datacatalog_v1beta1.DataCatalogClient()
info = client.lookup_entry(
    sql_resource="bigquery.my-project.schema_examples.array_struct"
)
print(info)

[tswast]

FYI: This client is 100% auto-generated.

[busunkim96]

LGTM.

To get the docs to appear there are handful more steps (copied from Configure Docs in the Python Cloud Client Libraries Playbook).

1. Create a link between docs and the service docs

   cd docs                                                                                                                                                             
   ln -s ../SERVICE/docs SERVICE
   

2. Edit docs/requirements.txt to include this service
3. Edit docs/index.rst to include it in ToC.

[tswast]

Thanks for the reminder re: docs. I should really use that playbook as a checklist.